GIFT  OF 
Professor  W.  S.  Morley 


Engineering  Library 


April  7,  1958 


DYNAMO  ELECTRIC  MACHINERY; 


ITS   CONSTRUCTION,    DESIGN, 
AND   OPERATION 


DIRECT    CURRENT   MACHINES 


BY 

SAMUEL    SHELDON,  A.M.,  PH.D. 

PROFESSOR   OF    PHYSICS    AND   ELECTRICAL    ENGINEERING    AT    THE    POLYTECHNIC 

INSTITUTE   OF    BROOKLYN,    MEMBER    OF    THE    AMERICAN    INSTITUTE 

OF  ELECTRICAL   ENGINEERS,    AND    FELLOW    OF    THE    AMERICAN 

ASSOCIATION    FOR    THE    ADVANCEMENT   OF   SCIENCE 

ASSISTED    BY 


HOBART    MASON,  B.S. 


NEW   YORK: 
D.   VAN    NOSTRAND    COMPANY 

23  MURRAY  AND  27  WARREN  STS. 
1900 


COPYRIGHT,  1900,  BY 
D.  VAN  NOSTRAND   COMPANY 


\  - 


^v    \JO 

A 


PREFACE. 


THIS  book  is  intended  to  be  used  primarily  in  connec- 
tion with  instruction  on  courses  of  electrical  engineering  in 
institutions  for  technical  education.  It  is  laid  out  on  the 
lines  of  the  lectures  and  the  instruction  as  given  in  the 
Polytechnic  Institute  of  Brooklyn.  It  is  intended  equally 
as  much  for  the  general  reader,  who  is  seriously  looking 
for  information  concerning  dynamo  electrical  machinery  of 
the  types  discussed,  as  well  as  a  book  of  reference  for 
engineers. 

The  first  two  chapters  are  devoted  to  a  brief  but  logical 
discussion  of  the  electrical  and  magnetic  laws  and  facts 
upon  which  the  operation  of  this  class  of  machinery 
depends.  Calculus  methods  have  been  employed  in  a  few 
places  in  these  chapters,  but  the  results  arrived  at  by  use 
of  them  are  in  such  a  form  that  they  can  be  utilized  by  the 
reader  who  is  unfamiliar  with  the  processes  of  the  calculus. 

In  the  chapter  on  design  it  has  seemed  advisable  to 
express  the  flux  density  in  lines  per  square  centimeter. 
Both  the  square  centimeter  and  the  square  inch  are  used 
in  practice.  The  alteration  of  the  formulas  to  square  inch 
units  is  obviously  simple. 

We  wish  to  express  our  thanks  to  the  various  manufac- 
turing companies  who  have  so  courteously  given  informa- 
tion, and  who  have  kindly  loaned  electrotypes  of  their 
apparatus. 

1)90644 


CONTENTS. 


CHAPTER  PAGE 

I.  ELECTRICAL  LAWS  AND  FACTS i 

II.  MAGNETIC  LAWS  AND  FACTS 12 

III.  ARMATURES 31 

IV.  FIELD  MAGNETS 67 

V.  OPERATION  OF  ARMATURES 77 

VI.  EFFICIENCY  OF  OPERATION 92 

VII.  CONSTANT  POTENTIAL  DYNAMOS 103 

VIII.  CONSTANT  CURRENT  DYNAMOS 129 

IX.  MOTORS 161 

X.  SERIES  MOTORS 185 

XI.  DYNAMOTORS,  MOTOR-GENERATORS,  AND  BOOSTERS      .    .  208 

XII.  MANAGEMENT  OF  MACHINES 218 

XIII.  THE  DESIGN  OF  MACHINES 232 

XIV.  TESTS 251 


DYNAMO  ELECTRIC  MACHINERY. 


CHAPTER   T.  .V:..H' ••''•'?'5' 

ELECTRICAL    LAWS    AND    FACTS. 

i.    Mechanical  Units Force  is  that   which    tends  to 

produce,  alter,  or  destroy  motion.  The  units  of  force  are 
the  pound  and  the  dyne.  The  dyne  is  that  force,  which 
acting  on  one  gram  for  one  second,  will  produce  a  velocity 
of  one  centimeter  per  second. 

Work  is  the  production  of  motion  against  resistance. 
The  units  of  work  are  the  foot-pound  and  the  erg.  The 
foot-pound  is  the  work  done  in  lifting  a  body  weighing  one 
pound  one  foot  vertically.  The  erg  is  the  work  performed 
by  a  force  of  one  dyne  in  moving  a  body  one  centimeter 
in  the  direction  of  its  acting.  The  joule  is  a  larger  unit 
much  used,  and  is  equal  to  io7  ergs. 

Energy  is  the  capacity  to  do  work.  Energy  is  divided 
into  Kinetic  energy  and  Potential  energy.  A  body  pos- 
sesses kinetic  energy  in  virtue  of  its  motion,  while  poten- 
tial energy  is  due  to  the  separation  or  the  disarrangement 
of  attracting  particles  or  masses.  A  wound  up  spring  has 
potential  energy  because  of  the  strained  positions  of  the 
molecules,  while  a  weight  raised  to  a  height  has  potential 
energy  because  of  the  separation  of  its  mass  from  the 


2  DYNAMO   ELECTRIC   MACHINERY. 

attracting  mass  of  the  earth.  The  potential  energy  of  a 
body  is  measured  by  the  work  required  to  put  the  body 
into  its  strained  condition.  Kinetic  energy  is  measured  by 
the  product  of  the  weight  into  the  square  of  the  velocity 
divided  by  twice  the  acceleration  due  to  gravity,  or 

Kinetic  Energy  = 


'?.'';        *e  /,':.;•'  2g 

'  'Power  '{§  «the^  fcate  of  performance  of  work.     Its  units 

lar^  tfta'hp'rEd-ptfw£r  and  the  watt.  A  horse-power  is  33,000 
foot-pounds  per  'minlite.  A  watt  is  io7  ergs  per  second. 
One  horse-power  is  equivalent  to  746  watts.  The  number 
of  watts  in  an  electrical  circuit  carrying  a  certain  number  of 
amperes  of  current  under  a  pressure  of  a  certain  number 
of  volts  is  expressed  by  the  product  of  the  amperes  into  the 
volts.  If  we  let  T  equal  the  torque  or  twisting  moment 

QTTH 

and  w  equal  the  angular  velocity  (=  -  where  n  is  the 

60 

number  of  revolutions  per  minute),  then  the  horse-power 

_  6ow7T_  zirnT 
33000      33000  * 

In  a  belt-driven  machine  the  torque  in  the  shaft  is  equal 
to  the  difference  in  tension  of  the  two  sides  of  the  belt 
multiplied  by  the  radius  of  the  pulley  in  feet,  hence 


2.  Absolute  and  Practical  Units  --  Since  distinction 
must  continually  be  made  between  the  absolute  units  and 
the  practical  units,  throughout  this  work  the  capital  letters 
/,  E,  and  R  will  be  used  for  the  practical  units,  the  am- 
pere, the  volt,  and  the  ohm,  respectively,  and  the  lower- 
case letters  i,  e,  and  r  will  stand  for  the  absolute  (C.  G.  S.) 
units  of  current,  pressure,  and  resistance  respectively. 


ELECTRICAL   LAWS   AND    FACTS.  3 

The  absolute  unit  of  current  is  such  that,  when  flowing 
through  a  conductor  of  one  centimeter  length,  which  is 
bent  into  an  arc  of  one  centimeter  radius,  it  will  exert  a 
force  of  one  dyne  on  a  unit  magnet  pole  (§  10)  placed  at  the 
center. 

The  absolute  unit  of  difference  of  potential  exists  between 
two  points  when  it  requires  the  expenditure  of  one  erg  of 
work  to  move  a  unit  quantity  of  electricity  from  one  point 
to  the  other.  This  unit  of  quantity  is  the  quantity  which, 
in  a  second,  passes  any  cross-section  of  a  conductor  in 
which  a  unit  current  is  flowing. 

The  absolute  unit  of  resistance  is  offered  by  a  body  when 
it  allows  a  unit  current  to  flow  along  it  between  its  two 
terminals,  when  maintained  at  a  unit  difference  of  potential. 

Current,  f=   — /. 
10 

E.M.F.,  E  =  io»e. 
Resistance,  JR  =  loV. 

It  is  convenient  and  rational  to  make  a  distinction  be- 
tween electromotive  force  and  difference  of  potential. 
Electromotive  force  is  produced  when  a  conductor  cuts 
magnetic  lines  of  force,  or  when  the  electrodes  of  a  pri- 
mary battery  are  immersed  in  a  solution.  But  a  difference 
of  potential  may  exist  merely  because  of  an  electric  cur- 
rent. Between  any  two  points  of  a  conductor  carrying  a 
current  there  is  that  which  would  send  a  current  through 
an  auxiliary  wire  connecting  these  points,  and  we  call  it 
difference  of  potential.  If  the  current  in  the  original  con- 
ductor be  doubled,  the  difference  of  potential  between  the 
same  two  points  will  be  doubled,  showing  that  this  differ- 
ence of  potential  exists  because  of  the  current  flowing  in 


4  DYNAMO   ELECTRIC   MACHINERY. 

the  original  conductor.  The  word  pressure  is  used  for 
either  difference  of  potential  or  for  E.M.F.  with  obvious 
relevancy. 

3.   Ohm's  Law  --  Ohm's  law  is  expressed  by  the  for- 
mula r, 


where  7  is  the  number  of  amperes  flowing  in  an  undivided 
circuit,  E  the  algebraic  sum  of  all  the  electromotive  forces 
in  that  circuit,  and  R  the  sum  of  all  the  resistances  in 
series  in  that  circuit. 

The  form  of  the  equation  E  =  IR,  as  applied  to  a  por- 
tion of  a  circuit,  is  much  used  under  the  name  of  Ohm's 
law.  In  this  case,  however,  E  is  not  E.M.F.,  but  differ- 
ence of  potential,  as  explained  in  the  last  article. 

If,  in  a  house  lighted  by  electricity,  the  service  maintains 
a  constant  pressure  of  100  volts  at  the  mains  where  they 
enter  from  the  street,  and  no  lights  be  turned  on,  then  at 
every  lamp  socket  in  the  house  there  will  be  a  pressure  of 
100  volts.  If  now  a  lamp  be  turned  on,  it  will  be  working 
on  less  than  100  volts,  because  of  the  drop  or  fall  of  po- 
tential. If  many  lamps  be  turned  on,  a  considerable  drop 
may  occur.  The  drop  is  caused  by  the  resistance  of  the 
wires  carrying  the  current  from  the  place  of  constant  po- 
tential to  the  place  where  it  is  used,  and  the  volts  lost  have 
been  consumed  in  doing  useless  work  in  heating  the  wires. 
That  the  drop  is  proportional  to  the  current  flowing  is 
shown  by  a  simple  application  of  Ohm's  law. 

Let  R  be  the  resistance  of  the  line,  and  Ed  the  volts 
drop  caused  thereby  when  a  current  /  flows.  Then 

E*  =  IR* 

from  which  it  is  evident  that  the  drop  varies  as  the  cur- 
rent when  the  resistance  in  the  line  is  constant. 


ELECTRICAL  LAWS  AND  FACTS.         5 

4.  Resistance  of  Conductors. — The  resistance  R  of  a  con- 
ductor is  expressed  by  the  formula  R  =  — ,  where  o-  is  a 

A 

constant  called  the  resistivity,  and  depending  upon  the 
material  and  the  temperature  of  the  conductor,  /  is  the 
length  in  centimeters,  and  A  the  cross-section  in  square 

cms.  The  reciprocal  of  the  resistivity,  — ,  is  called  the  con- 
ductivity of  a  substance. 

The  conductivity  of  copper  depends  on  its  purity,  and  on 
its  physical  condition,  soft  copper  having  1.0226  times  the 
conductivity  of  hard  copper.  Lake  copper  has  a  high  con- 
ductivity because  of  its  pureness.  The  same  is  true  of 
electrolytic  copper.  This  latter  is  now  very  largely  used, 
though  for  a  while  there  was  a  prejudice  against  it  because 
it  was  said  to  be  brittle.  Temperature  affects  the  resist- 
ance of  metals.  In  pure  metals  the  increase  of  resistance 
for  a  rise  of  i°  C.  is  about  .004  times  their  resistance  at 
o°  C.  Various  alloys  of  iron,  nickel,  and  manganese  have 
a  high  value  for  a-,  and  do  not  have  so  high  a  temperature 
coefficient  as  given  above.  Iron  heated  in  contact  with 
copper  gives  a  large  thermal  E.M.F.,  which  militates  against 
its  alloys  being  used  for  resistances  in  measuring  instru- 
ments. 

If  in  the  foregoing  expression  for  R  the  centimeter  and 
square  centimeter  be  the  units  of  length  and  cross-section 
respectively,  then  the  following  list  gives  the  value  of  o-  for 
various  metals  in  microhms  (i  microhm  =  TTnJV<yoff  ohm). 

Copper    ....  at  o°C, I-594 

Iron    .'....""        9.5 

Steel "     "        13.0 

18%  German  Silver  "      "        27. 

30% "      "        45- 


6  DYNAMO   ELECTRIC   MACHINERY. 

A  circular  mil  is  a  circle  j-^^  inch  in  diameter,  and  a 
wire  one  foot  long  and  one  circular  mil  cross-section  is 
called  a  mil-foot.  The  resistance  of  a  mil-foot  at  o°  C.  of 

Copper  =  9.59  ohms, 
Iron  =58.  ohms, 
Steel  =82.  ohms. 

The  American  Institute  of  Electrical  Engineers  has 
adopted  as  its  standard  resistivity  for  soft  copper  one  given 
by  Matthiessen.  A  wire  of  standard  soft  copper,  of  uni- 
form cross-section,  of  one  meter  length,  and  weighing  one 
gram,  should  have  a  resistance  of  0.141729  international 
ohms  at  o°  C.  A  commercial  copper  showing  this  resis- 
tivity is  said  to  have  100  per  cent  conductivity.  Copper  is 
frequently  found  having  a  conductivity  of  102  per  cent.  It 
is  in  these  cases  almost  invariably  electrolytic  copper. 

5.  Insulating  Materials.  —  Materials  which  are  to  be 
used  for  insulating  from  each  other  the  various  electrical 
circuits  of  dynamo  electric  machines  should  possess  the 
following  properties :  — 

They  should  have  a  high  insulation  resistance,  and  this 
resistance  should  be  maintained  high  over  the  range  of 
temperatures  to  be  found  in  machines.  They  should 
furthermore  have  a  dielectric  strength  sufficient  to  pre- 
clude any  possibility  of  their  being  perforated  by  the 
voltages  liable  to  exist  between  the  conductors  which  they 
separate.  This  strength  must  also  exist  throughout  all 
probable  ranges  of  temperature.  They  must  possess  such 
physical  properties  as  will  permit  of  mechanical  manipula- 
tion, as  they  must  be  oftentimes  bent  and  twisted.  Of 
course  the  chemical  constitution  should  not  be  altered  by 


ELECTRICAL  LAWS   AND   FACTS.  7 

any  change  of  temperature  to  which  they  would  be  sub- 
mitted. 

Mica  possesses  the  highest  insuktion  resistance  and  the 
largest  dielectric  strength  to  be  found.  It  requires  1000 
volts  to  perforate  a  sheet  I  mil  in  thickness.  Its  chemical 
constitution  is  unaffected  by  high  temperatures.  It  is 
not,  however,  mechanically  strong. 

Preparations  of  fibrous  materials  with  linseed  oil,  which, 
after  being  dried,  have  been  thoroughly  baked,  are  fairly 
good  insulators.  As  water  is  generally  present  in  their 
pores,  their  insulation  resistance,  upon  heating,  decreases 
until  the  temperature  has  reached  100°  C,  and  then  it  in- 
creases. These  preparations  are  mechanically  flexible. 
Preparations  of  fibrous  material  with  shellac  are  good  in- 
sulators, but  crack  upon  bending. 

Vulcanized  fibers  are  made  by  treating  paper  fiber  chemi- 
cally, and,  when  dried,  they  have  a  fairly  high  insulation 
resistance,  but  they  readily  absorb  moisture,  and,  upon  dry- 
ing, are  liable  to  warp  and  twist.  They  furthermore  be- 
come brittle  when  heated. 

Sheets  of  insulation  made  up  from  pieces  of  scrap  mica 
cemented  together  by  linseed  oil  or  preparations  of  shellac, 
when  carefully  constructed  with  lapped  joints,  exhibit 
nearly  as  good  insulating  and  dielectric  properties  as  sheet 
mica.  While  not  perfect  mechanically,  these  sheets  permit 
of  bending  better  than  pure  mica. 

Vulcabeston,  which  is  a  preparation  of  asbestos  and  rub- 
ber, exhibits  fairly  good  insulating  and  mechanical  quali- 
ties, and  is  especially  fitted  for  higher  temperatures.  Its 
dielectric  strength  is  about  TJ^  of  that  of  mica. 


6.   Divided  Circuits  --  If  a  current  I  be  flowing  through 


8  DYNAMO   ELECTRIC   MACHINERY. 

Ry  the  undivided  part  of  the  circuit  shown  in  Fig.  I,  and 
if  7j  and  72  be  the  currents  flowing  in  the  shunt  resistances 
RI  and  Ry  then  /  =  7X  +  Iv  and,  since  the  pressure  E 
upon  each  shunt  is  the  same,  by  Ohm's  law, 


or  Ii  :  72  :  :  —  :  — 

<«1       ^2 

currents  in  the  branches  of  a  divided  circuit  are  in- 
versely as  the  resistances  of  the  branches. 

If  Re  be  a  single  resistance,  that  substituted  for  the 
shunted  resistances  Rl  and  Rz  will  leave  7  unchanged,  then, 
by  Ohm's  law, 


resistance  equivalent  to  a  number  of  shunted  resistances 
is  equal  to  the  reciprocal  of  the  sum  of  the  reciprocals  of  the 
separate  resistances. 

RI 


1 

I 


00  U  O U  U 


Fig.  x. 

7.  Power  of  Electric  Current.  —  A  difference  of  poten- 
tial e  between  two  points  requires  e  ergs  of  work  to  bring 


ELECTRICAL   LAWS   AND   FACTS.  9 

a  unit  quantity  of  electricity  from  one  point  to  the  other. 
A  unit  of  quantity  is  one  absolute  unit  of  current  flowing 
for  one  second.  Hence  a  current  i  flowing  for  /  seconds  with 
a  difference  of  potential  e  does  eit  ergs  of  work.  Likewise 
a  current  /  flowing  /  seconds  gives  It  coulombs  of  quantity, 

Eit 
and  with  a  difference  of  potential  of  E  volts  does  —  r  ergs 

10     El 
of  work.     Hence  the  work  per  second  or  the  power  is  —  7 

absolute  units.  The  practical  unit  of  power  is  the  watt, 
and  equals  io7  absolute  units.  Hence,  remembering  that 
by  Ohm's  law  E  =  IR,  the  power  of  a  current  in 


For  commercial  currents  and  voltages  the  watt  is  a  need- 
lessly small  expression,  hence  the  kilowatt  (=  1,000  watts) 
is  generally  used  as  the  unit  of  electrical  power.  It  is  repre- 
sented by  the  abbreviation  K.  w.  The  horse-power  is 
equal  to  746  watts,  or  approximately  three-fourths  of  a  K.W. 

8.  Heat  Developed  by  a  Current  --  When  a  current  / 
does  work  in  overcoming  a  resistance  R,  the  work  per- 
formed is  converted  into  heat.  By  the  last  article  the 
work  thus  done  per  second,  or  the  power  expended,  will  be 
PR  watts.  Since  this  rate  of  production  of  heat  is  often 
of  no  service,  this  expenditure  of  power  is  generally  called 
the  PR  loss. 

This  production  of  heat  causes  a  rise  of  temperature  in 
the  conductor,  and  the  temperature  will  continue  to  rise  till 
the  heat  generated  per  second  by  the  PR  loss  is  exactly 
counterbalanced  by  the  rate  of  dissipation  of  heat  by  con- 
duction, convection,  and  radiation. 

The  necessary  resistances  of  electrical  machines  involve 


10  DYNAMO   ELECTRIC   MACHINERY. 

the  production  of  heat  in  their  operation  (as  does  also  fric- 
tion and  reversal  of  magnetism),  which  causes  a  rise  of  tem- 
perature. As  insulating  materials  can  survive  only  moder- 
ately high  temperatures,  such  machines  must  be  designed 
to  operate  without  becoming  too  hot.  This  is  accomplished 
by  decreasing  the  PR  loss,  by  increasing  the  radiating  sur- 
face, and  by  supplying  ventilation. 

9.  Fuses.  —  These  are  devices  intended  to  protect  cir- 
cuits from  destruction  or  damage  due  to  an  excessive  flow 
of  current  through  them.  They  protect  them  by  being 
themselves  destroyed.  They  are  generally  made  of  lead 
or  alloys  of  lead.  Lead  is  liable  to  become  oxidized  after 
having  been  installed  for  some  time.  It  is  then  liable  to 
form  a  tube  of  hard  oxide,  which  is  sufficiently  strong  to 
hold  molten  lead  in  its  interior,  so  as  to  maintain  an  elec- 
trical contact  in  the  circuit  which  should  be  broken.  Some 
alloys  are  not  open  to  this  objection.  These  alloys,  in 
the  form  of  wires,  strips,  or  ribbons,  are  fastened  at  each 
end  to  copper  terminals  which  are  slotted  to  fit  into  fuse 
receptacles.  The  wire  with  its  terminals  is  called  a  fuse 
link.  Such  a  link  is  shown  in  Fig.  2. 


Fig.  2. 

Copper  wires  are  sometimes  used  as  fuses  on  trolley  cars, 
but  the  high  melting  point  of  copper  prohibits  its  use  as  a 
protective  device  on  house  circuits. 

The  current  which  will  fuse  a  wire  of  lead  alloy  depends 
in  magnitude  upon  the  length  of  the  wire.  Short  lengths 


ELECTRICAL  LAWS    AND   FACTS.  II 

of  a  wire  of  given  cross-section  and  given  material  will 
carry  stronger  currents  than  longer  lengths.  The  heat 
which  is  generated  in  the  short  ones  escapes  more  rapidly, 
owing  to  the  larger  masses  of  metal  commonly  forming  the 
terminals  of  the  fuse.  Fuses  are  rated  to  carry  a  given 
amperage,  and  the  rating  is  stamped  upon  the  copper  termi- 
nals. According  to  the  national  code  the  fuses  must,  how- 
ever, be  able  to  carry  indefinitely  without  melting  such  a 
number  of  amperes  that  the  rated  capacity  is  but  80  per 
cent  of  it.  This  permits  the  fuse  to  carry  without  melting 
25  per  cent  above  the  rated  capacity. 


Fig.  3. 


For  high  voltages,  and  for  large  amperages,  inclosed 
fuses  are  sometimes  used,  in  which  the  fusible  conductor  is 
surrounded  by  a  packing  of  finely  divided  powder  in  which 
borax  is  included  as  an  element  most  desirable.  Such  a 
fuse  is  shown  in  Fig.  3. 


12  DYNAMO   ELECTRIC   MACHINERY. 


CHAPTER    II. 

MAGNETIC    LAWS   AND   FACTS. 

10.  Strength  of  Magnet  Pole A  unit  magnet  pole  is 

one  which  will  repel  an  equal  like  pole,  when  at  a  distance 
of  one  centimeter,  with  a  force  of  one  dyne. 

It  follows  from  this  definition  that  a  pole  m  units  strong 
will  repel  a  like  unit  pole  with  a  force  of  m  dynes.  The 
force  exerted  between  two  magnetic  poles  varies  inversely 
as  the  square  of  the  distance  between  them.  Hence  the 
force  exerted  between  two  magnetic  poles  of  strengths  m 
and  m'  when  d  centimeters  apart  is  defined  by  the  equation 

m  m' 

F"    ~d^' 

11.  Intensity  of   Magnetic  Field.  —  A  magnetic  field  is 
of  unit  strength  or  intensity  when  a  unit  magnet  pole  placed 
therein  is  acted  upon  by  a  force  of  one  dyne,  or  when  a 
magnet  pole  m  units  strong  is  acted  upon  by  a  force  of 
m  dynes.     The  strength  of  a  field  is  usually  represented 
by  3C. 

12.  Magnetic   Field  and  Lines   of   Force.  —  The  space 
around  a  magnet  where  its  action  is  felt  is  termed  the  field 
of  that   magnet.     This   field  may  conveniently  be  consid- 
ered as  permeated  by  lines  of  force.     These  lines  represent 
the  direction  of  the  force  exerted  by  the  magnet,  and  by 
their  closeness  to  each  other  show  the  magnitude  of  this 
force. 


MAGNETIC   LAWS   AND    FACTS.  13 

The  student  must  not  get  the  impression  that,  because 
the  lines  spread  apart,  a  point  in  the  field  could  be  chosen 
where  there  would  be  no  line.  These  lines  may  well  be 
considered  as  tubes  or  pencils  of  force,  filling  all  the  space 
around  the  magnet. 

The  lines  of  force  contained  in  any  plane  passed  through 
the  magnet  pole  compose  a  magnetic  spectrum,  which  can 
be  made  visible  by  the  familiar  experiment  of  sprinkling 
iron  filings  on  a  paper,  which  is  laid  over  a  magnet,  and  by 
gently  tapping  it. 

By  convention  one  line  of  force  per  square  centimeter  is 
considered  to  represent  a  field  of  unit  strength,  the  square 
centimeter  being  so  taken  that  it  is  at  all  points  perpendic- 
ular to  the  lines  cutting  it.  Hence  the  strength  or  inten- 
sity 3C  of  any  field  can  be  expressed  by  the  number  of 
lines  of  force  per  square  centimeter. 

Suppose  a  sphere  of  one  centimeter  radius  to  be  circum- 
scribed about  a  unit  magnet  pole.  Another  unit  pole  at 
any  point  on  the  surface  of  this  sphere  will  be  acted  upon 
by  a  force  of  one  dyne.  Hence  there  exists  a  unit  field  at 
any  point  on  this  surface.  But  there  are  4  -n-  square  centi- 
meters on  this  surface,  and  each  square  centimeter  will 
be  cut  by  one  line  of  force.  Therefore,  there  emanate 
from  a  tmit  magnet  pole  4  TT  lines  of  force.  Similarly  a 
magnet  pole  of  strength  m  sends  out  4  TT  m  lines  of  force. 

A  magnetic  field  is  said  to  be  uniform  when  it  has  the 
same  3C  at  every  point  therein,  or  when  the  lines  of  force 
are  parallel. 

13.  Electro-Magnetic  Induction.  —  In  1831  Faraday  and 
Henry  independently  discovered  that  when  a  conductor  was 
moved  in  a  magnetic  field,  an  electromotive  force  was  set 


DYNAMO    ELECTRIC    MACHINERY. 


up  in  the  conductor.  This  phenomenon  is  the  foundation 
of  all  modern  electrical  engineering. 

An  absolute  unit  of  E.M.F.  is  produced  when  a  conduc- 
tor cuts  one  line  of  force  per  second.  If  the  conductor 
cuts  two  lines  in  the  second,  or  one  line  in  half  a  second, 
then  two  units  are  produced. 

If,  in  the  short  interval  of  time,  dt  seconds,  dfy  lines  be 
cut,  then  during  that  interval  the  value  of  the  induced 
E.M.F.  will  be 


_ 
~ 


dt 


or, 


=  __8       voits. 
10   dt 


The  negative  sign  is  used  because  the  induced  E.M.F. 
tends  to  send  a  current  in  such  a  direction  as  to  demag- 

netize the  field.  When  of  no  con- 
sequence the  negative  sign  will 
hereafter  be  omitted. 

If  a  conductor,  Fig.  4,  /  centi- 
meters long  moves  parallel  to  itself 
with  a  uniform  velocity  of  v  cen- 
timeters per  second  across  a  uni- 
form magnetic  field  of  strength 
3C,  its  path  making  an  angle  a  with 
the  direction  of  the  lines  of  force, 
then  the  number  of  lines  cut  per 
second  is  3£lv  sin  a,  and  since  the 
rate  of  cutting  is  uniform,  the  E.M.F.  at  any  instant  is 

e  =  3£lv  sin  a. 

If  there  be  a  non-uniformity  in  the  rate  of  cutting  lines, 
due  either  to  an  uneven  field  or  an  irregular  motion,  then 


Fig.  4- 


MAGNETIC   LAWS    AND    FACTS.  15 

the  average  value  of  the  induced  E.M.F.  associated  with 
the  cutting  of  <£  lines  in  the  time,  t  seconds,  will  be  eav  =  -• 

For  suppose  the  time  t  to  be  divided  into  /  equal  and  small 
periods  havirg  a  duration  of  A/  seconds.  Furthermore, 
suppose  that  during  these  successive  periods  A<£',  A<£",  A<£'", 
etc.,  lines  be  cut  respectively.  Then  the  induced  E.M.F.'s 
during  these  periods,  which  may  be  represented  by  /,  /',  e' 
etc.,  respectively,  will  be  as  follows :  — 

/  =  ^r 

e"  =  ~KT ' 


'" 


A/ 


Adding  these  /  equations,  and  then  dividing  by  /,  gives  the 
equation  above,  viz., 

'-  =  7  or  £-  =  ^volts' 

The  average  value  of  the  induced  E.M.F.  is  therefore  inde- 
pendent of  the  magnitude  of  the  instantaneous  values. 

If  a  loop  of  wire  revolve,  uniformly  or  otherwise,  in  a 
magnetic  field  which  is  uniform  or  otherwise,  its  sides  cut 
lines  of  force  at  various  rates.  The  instantaneous  E.M.F. 
in  the  whole  loop  will  be  as  before. 


where  <£  is   the  number  of  lines  that  links   with,  or  that 
passes  through,  the  loop.     If  the  loop  be  of  n  turns,  then 


i6 


DYNAMO   ELECTRIC   MACHINERY. 


the  pressure  will   be  n  times  as  great,  or  during  the  inter- 
val dt, 


14.  Direction  of  Induced  E.M.F.  —  The  direction  of 
flow  of  a  current  induced  in  a  closed  circuit  by  mov- 
ing it  in  a  magnetic  field  is  best  represented  by  drawing 
the  conventional  representation  of  the  three  dimensions 
of  space.  If  the  flux  be  directed  upwards,  and  the  motion 
of  the  conductor  be  to  the  right,  then  the  E.M.F.  will  tend 
to  send  a  current  toward  the  reader.  If  any  one  of  these 
conditions  be  changed  it  necessitates  the  change  of  one  of 
the  others,  and  conversely  the  change  of  any  two  leaves 


I 


Moiior 


Fig.  5. 


Fig.  6. 


the  third  unaltered.     About  the  same  idea  is  represented 
in  Fleming's  Rule,  which  is  as  follows  :  — 

Let  the  index  finger  of  the  right  hand  point  in  the  di- 
rection of  the  flux,  and  the  thumb  in  the  direction  of  the 


MAGNETIC   LAWS   AND    FACTS.  I/ 

motion.  Bend  the  second  finger  at  right  angles  with  the 
thumb  and  index  finger,  and  it  will  point  in  the  direction 
of  the  E.M.F. 

Another  rule  is  :  — 

Stand  facing  a  north  magnetic  pole.  Pass  a  conductor 
downward.  The  current  tends  to  flow  to  the  left. 

15.  Inductance.  —  Nearly  every  electrical  circuit  which 
has  a  current  flowing  in  it  has  lines  of  force  linked  with  it, 
due  to  that  current.  When  the  circuit  is  opened  the  dis- 
appearance of  the  lines  is  accompanied  by  a  cutting  of  the 
circuit  by  those  lines,  and  the  cutting  results  in  the  pro- 
duction of  an  E.M.F.  This  is  called  the  E.M.F.  of  self- 
induction.  Its  magnitude  is  dependent  upon  the  rapidity 
with  which  the  field  disappears,  and  upon  a  constant  deter- 
mined by  the  geometric  shape  of  the  circuit  and  the  char- 
acter of  the  medium  in  which  it  is  placed.  This  constant 
is  called  the  self-inductance  or  the  coefficient  of  self-induc- 
tion of  the  circuit.  It  is  generally  represented  by  the 
letter  Z,  and  is  that  coefficient  by  which  the  time  rate  of 
change  of  current  in  the  circuit  must  be  multiplied  in  order 
to  give  the  E.M.F.  induced  in  the  circuit.  Its  absolute 
value  is  numerically  represented  by  the  number  of  lines  of 
force  linked  with  the  circuit  per  absolute  unit  of  current  in 
that  circuit.  Its  practical  unit  is  io9  times  as  large  as  the 
absolute  unit,  and  is  called  the  henry.  In  a  given  circuit  it 
varies  as  the  square  of  the  number  of  turns  of  wire.  Two 
circuits  may  exercise  a  mutually  inductive  action  upon  each 
other,  and  an  E.M.F.  may  be  induced  in  one  by  a  change 
of  current  in  the  other.  This  is  called  the  E.M.F.  of 
mutual  induction.  In  magnitude  it  depends  upon  the  shape 
and  position  of  the  two  circuits,  and  upon  the  character 


18  DYNAMO   ELECTRIC   MACHINERY. 

of  medium  in  which  they  are  placed.  It  is  also  dependent 
upon  a  constant  which  is  called  the  mutual  inductance  or 
coefficient  of  muttial  induction  of  the  two  circuits.  It  is 
generally  represented  by  the  letter  M.  It  is  that  coeffi- 
cient by  which  the  time  rate  of  change  of  the  current  in  one 
of  the  circuits  is  multiplied  in  order  to  give  the  E.M.F. 
induced  in  the  other  circuit.  Its  absolute  value  is  numeri- 
cally equal  to  the  number  of  lines  of  force  linked  with  one 
of  the  circuits  per  absolute  unit  of  current  in  the  other  cir- 
cuit. Its  practical  unit  is  the  same  as  the  practical  unit 
of  self  -inductance,  that  is  the  henry,  and  is  io9  times  as 
large  as  the  absolute  unit.  The  coefficient  of  mutual  in- 
duction varies  directly  as  the  number  of  turns  of  wire  in 
either  circuit. 

16.  Quantity  of  Electricity  Traversing  a  Circuit  Due 
to  a  Change  of  Flux  Linked  with  it.  —  In  many  dynamo 
tests,  and  in  many  magnetic  investigations,  it  is  necessary 
to  measure,  generally  by  means  of  a  ballistic  galvanometer, 
the  quantity  of  electricity  traversing  a  circuit  due  to  a 
change  of  flux  linked  with  it.  If  the  circuit  have  a  resist- 
ance of  r  and  in  dt  time  the  flux  linked  with  n  turns 
changes  by  d$,  then  the  instantaneous  current 


dt 

t  =  -  • 
r 

But  the  quantity,  q  =  idt,  hence 

ndd» 

?=—  • 

which  is  independent  of  time.      So  if  the  flux  change  from 
<j>:  to  <£2,  then 

9l   —    <P2 


MAGNETIC   LAWS   AND    FACTS.  19 

or  Q  = _.       microcoulombs. 

100        J? 

17.  Work  Performed  by  a  Conductor  Carrying  a  Current 
and  Moving  in  a  Magnetic  Field.  —  Let  a  conductor  carry- 
ing a  constant  current  i  be  moved  in  a  direction  perpen- 
dicular to  itself  and  to  the  lines  of  force  of  a  magnetic  field. 
Suppose  it  to  move  for  dt  seconds,  and  in  that  time  to  cut 
d$  lines  of  force.  Then  the  induced  E.M.F.  e  will  be 

-¥.     The  quantity  of  electricity  dq  that  has  to  traverse 

the  circuit  against  this  E.Af.F.  during  the  time  dt  will 
be  idt.  Since  potential  is  a  measure  of  work,  the  work 
required  to  carry  dq  units  of  electricity  against  a  difference 
of  potential  e  is  edq  ergs.  Hence  the  work  in  ergs, 

dw  —  edq  =  idt  X  -p  =  *>/<£. 

Therefore  the  current  /",  in  cutting  <£  lines  of  force,  per- 
forms the  work 

w  =  i(j>  ergs. 

From  this  it  is  seen  that  the  work  done  by  a  conductor 
carrying  a  current  and  cutting  lines  of  force  is  independent 
of  the  time  it  takes  to  cut  them. 

In  the  above  discussion,  if  the  field  be  not  uniform  or 
the  motion  be  not  uniform,  the  value  of  e  will  not  be  the 
same  for  each  instant  of  time.  But  since  the  result 
obtained  is  independent  of  time,  it  is  immaterial  how  the 
lines  are  arranged,  and  how  the  rate  of  cutting  varies. 

18.  Magnetic  Potential. — The  magnetic  potential  at 
any  point  is  measured  by  the  work  required  to  bring  a  unit 
magnet  pole  up  to  that  point  from  an  infinite  distance. 


20  DYNAMO   ELECTRIC   MACHINERY. 

The  difference  of  magnetic  potential  between  any  two 
points  is  measured  by  the  work  in  ergs  required  to  carry  a 
unit  magnet  pole  from  one  to  the  other.  The  difference 
of  magnetic  potential  is  a  measure  of  the  ability  to  send 
out  lines  of  force,  or  to  set  up  a  magnetic  field. 

19.  Magnetomotive  Force  of  a  Circular  Circuit  Carry- 
ing a  Current.  —  A  thin  circular  conductor  carrying  a  cur- 
rent forms  a  magnetic  shell.  If  a  unit  magnet  pole  be 
taken  from  the  top  side  of  a  shell,  and  carried  around  to 

the  bottom  side,  work  must  be 
done,  and  this  work  is  a  measure 
of  the  difference  of  potential  be- 
tween the  two  sides  of  the  shell. 
It  is  immaterial  whether  the 
pole  be  carried  from  one  side  of 
the  shell  to  the  other,  or  the 
shell  be  turned  bottom  side  up 

around  the  pole.  In  the  latter  case  it  is  clear  that  all  the 
lines  emanating  from  the  pole  will  be  cut  once,  and  once 
only,  by  the  conductor,  wherefore  4^  lines  will  have  been 
cut. 

If  current  i  flows  in  the  conductor,  then,  by  §  17, 

Work  in  ergs  /<£  =  4  TTZ. 

If  there  be  n  turns  of  the  conductor,  each  line  of  force 
will  be  cut  n  times,  and  the  work  will  be  ^irni  ergs. 

Hence  the  difference  of  potential  between  the  two  sides 

of  a  thin  magnetic  shell  is  4  imi  or       H   . 

10 

In  this  expression  -  -  is  a  constant,  and  it  is  convenient 
to  regard  nl  as  a  single  variable.  In  connection  with  it  the 


MAGNETIC   LAWS  AND   FACTS.  21 

term  ampere-turns  is  employed,  and  this  is  frequently 
written  ///. 

Here  the  same  argument  holds  as  in  §17,  that  the  inten- 
sity of  field  and  the  rate  of  cutting  lines  will  vary  as  the 
pole  is  in  different  parts  of  the  path.  But  the  total  num- 
ber of  lines  cut  is  the  same  in  any  case,  so  the  expression 
for  work  and  potential  is  true,  no  matter  what  path  the  pole 
takes. 

20.  Force  Exerted  on  a  Field  by  a  Conductor  Carrying  a 
Current When  a  conductor  moves  in  a  field  perpendicu- 
lar to  itself  and  to  the  lines  of  force,  then,  from  §  1 7, 

Work  =  /<£  ergs. 

If  the  conductor  be  /  centimeters  long,  and  traverses  a  dis- 
tance of  s  centimeters  through  a  uniform  field  of  strength 
3C  (3C  lines  per  sq.  cm.),  then 

<£  =  /tfC, 
and  the 

Work  =  Us  3C  ergs. 
But 

Work  =  force  X  distance  =  Fs  =  Us  3C. 

//TO 

.-.  ^=//X  =  =— -  dynes. 
10 

21.  The  Solenoid. — A  uniformly  wound,  long,  straight 
coil,  carrying  a  current  z,  produces  a  uniform  field  5C  at  its 
center.     This  coil  is  called  a  solenoid,  and  may  be  consid- 
ered as  composed  of  magnetic  shells  arranged  at  equal  dis- 
tances from  each  other.     It  takes  471-2'  ergs  to  move  a  unit 
magnet  pole  from  one  side  of  a  shell  to  the  other  (§  19), 
and  ^trin  ergs  to  pass  it  through  the  n  consecutive  shells 


22  DYNAMO   ELECTRIC   MACHINERY. 

of  the  solenoid.     If  these  n  shells  occupy  a  length  on  the 
solenoid  of  /  centimeters,  then 

Work  =  force  X  distance  =  3C/  =  ^-rrm  ergs, 

and  the  magnetizing  force,  that  is,  the  strength  or  intensity 
of  field,  in  the  solenoid  is 

OC  = 


22.  Permeability.  —  The  same  difference  of  magnetic 
potential  between  two  points  will  produce  more  lines  of 
force  in  iron  than  in  air.  Iron  is  then  said  to  be  more  per- 
meable than  air,  or  to  have  a  greater  permeability.  If  a 
difference  of  magnetic  potential  could  set  up,  at  a  certain 
place,  a  field  of  strength  3C,  with  air  as  a  medium,  and  one 
of  strength  <&,  with  some  other  substance  as  a  medium, 

/n 

then  the  ratio  —  expresses  the  permeability  of  that  sub- 
stance. This  ratio  is  usually  represented  by  /x.  As  3C 
varies  directly  with  the  magnetic  difference  of  potential,  it 
becomes  a  measure  of  it.  Therefore  3C  is  called  the  mag- 
netising force  and  <$>  the  flux  density,  the  magnetic  density, 
or  the  induction  per  square  centimeter.  For  air,  vacuum, 
and  most  substances  /*  =  i.  For  iron,  nickel,  and  cobalt  //. 
has  a  higher  value,  reaching,  in  the  case  of  iron,  as  high 
as  3000.  Bismuth,  phosphorus,  water,  and  a  few  other  sub- 
stances, have  a  permeability  very  slightly  less  than  unity. 

A  substance  for  which  ^  =  o  would  insulate  magnetism. 
There  is  no  such  substance. 

The  total  magnetic  flux,  <£,  which  passes  through  an 
area  of  A  square  centimeters,  in  which  the  magnetic  density 
is  ®,  is  represented  by  the  equation 


MAGNETIC   LAWS   AND   FACTS. 


The  permeability  of  air  is  constant  for  all  magnetizing 
forces.  This  is  not  the  case  with  iron  and  other  substances 
which  have  a  permeability  greater  than  unity.  The  value 
of  /A,  (&,  and  3C,  which  are  connected  by  the  relation  (&  =  /xX, 
are  given  in  the  following  table  for  average  commercial 
wrought  iron,  for  cast  iron,  and  for  steel.  The  relations 
which  exist  between  (B  and  3C  are  also  shown  in  Figs.  8,  9, 
and  10.  These  curves  are  technically  known  as  &-5C 
curves. 

DATA    FOR    (B-OC   CURVES. 

AVERAGE    FIRST   QUALITY    METAL. 


AMPERE- 

AMPERE- 

WROUGHT  AND 
SHEET  IRON. 

CAST  IRON. 

CAST  STEEL. 

5C 

PER  CEN- 

TURNS 
PER  INCH 

KILO- 

KILO- 

KILO- 

TIMETER 
LENGTH. 

LENGTH. 

(B 

LINES 

PER 

® 

LINES 

PER 

& 

LINES 

PER 

SQ.  IN. 

SQ.  IN. 

SQ.IN. 

10 

7-95 

2O.2 

11800 

74 

3900 

25.2 

I200O 

77 

20 

15.90 

40.4 

14000 

90 

55°° 

35-5 

13800 

89 

3° 

60.6 

15200 

98 

6500 

42.0 

I46OO 

94 

40 

31.80 

80.8 

15800 

102 

7100 

45-7 

15400 

99 

39-75 

IOI.O 

16400 

106 

7700 

49-5 

16000 

103 

60 

47.70 

121.  2 

16800 

1  08 

8200 

53-° 

16400 

106 

80 

100 

63.65 
79-5° 

161.6 

2O2.O 

17200 
17600 

III 

114 

8900 
9300 

57-2 
60.0 

l67OO 
I76OO 

1  08 
"3 

I25 

99.70 

252.5 

17800 

"5 

9700 

62.4 

I82OO 

117 

150 

119.25 

3°3-° 

18000 

116 

IOIOO 

65.8 

I8600 

1  20 

3C  =  i -258  (nl per  cm.)  =  .495  (nf  per  in.).    <&  =  . 1 55  (0  per  sq.  in.). 

23.  Things  Which  Influence  the  Shape  of  the  OHfC  Curve. 
—  In  general  all  substances  mixed  with  or  alloyed  with  iron 
lower  its  permeability.  In  steel  and  cast  iron  the  per- 
meability seems  to  be  in  inverse  proportion  to  the  amount 
of  carbon  present.  Carbon  in  the  graphitic  (not  combined) 
form  lowers  the  permeability  less  than  carbon  when  com- 
bined. In  cast  iron  and  cast  steel  such  substances  as  tend 
to  give  softness  and  greater  homogeneity  to  the  metal 


DYNAMO   ELECTRIC   MACHINERY. 


WROUGHT  AND  SHEET  IRON 


•B.J  per  centimeter  16 


10    20    30    40    50    60    70    80    90   100   110   120 


30 


a /per  inch  40 

Permeability^, 


_2_40. 


Fig.  8. 


45.5 


CAST  IRON 


MTpercentiint 


60         70         80        9C        100      110 


nl  per  incli 


Permeability//  100 


Fig.  9- 


MAGNETIC   LAWS   AND    FACTS. 


CAST  STEEL 


Fig.  10. 

when  present  in  limited  amounts,  say  2  per  cent,  increase 
the  value  of  /u.  Aluminum  and  silicon  act  in  this  way. 

The  physical  condition  of  the  metal  also  affects  its  per- 
meability. Chilling  in  the  mold,  when  casting,  lowers  it,  as 
does  tempering,  or  hardening  the  metal  by  working  it.  On 
the  other  hand,  annealing  increases  the  permeability. 

A  piece  of  iron  or  steel,  subjected  to  a  small  magnetizing 
force,  has  its  permeability  increased  by  increasing  the  tem- 
perature until  a  critical  temperature  is  reached,  when  it  falls 
off  rapidly  to  almost  jmity.  For  stronger  magnetization 
the  permeability  does  not  rise  so  high  at  the  critical  tem- 
perature, and  does  not  fall  off  so  sharply  after  it.  The 
value  of  this  critical  temperature  lies  between  650°  C.  and 
900°  C.,  depending  on  the  test  piece. 

24.  Reluctance  and  Permeance.  —  In  the  flow  of  mag- 
netic lines  of  force  the  reciprocal  of  the  permeability,  -,  is 


26  DYNAMO   ELECTRIC   MACHINERY. 

called  the  reluctivity.  The  total  reluctance,  tending  to 
oppose  the  passage  of  magnetic  lines  under  the  influence 
of  a  magnetic  difference  of  potential,  is  directly  as  the 
length  and  the  reluctivity  of  the  medium  and  inversely  as 
its  cross-section.  Hence  the  total  magnetic  resistance  or 

Reluctance  =  -  -  —  :  —  reluctivity. 
cross-section 

Reluctivity  is  usually  represented  by  p(=_).       Hence   for 

a  medium  of  cross-section  A  square  centimeters  and  length 
/  centimeters,  the  reluctance 


Permeance  is  the  reciprocal  of  the  reluctance,  hence  the 
permeance  ^  ^ 

*  =  *—/* 

It  must  be  remembered  that  p  and  p.  are  not  constant  for 
any  one  substance,  but  depend  for  their  values  upon  the 
strength  of  the  magnetizing  force  X  which  is  acting  upon 
the  substance. 

25.  Relation  Between  Magnetomotive  Force,  Magnetic 
Flux,  and  Reluctance.  —  These  quantities  are  related  to 
each  other  the  same  as  are  E.M.F.,  current,  and  resistance, 


viz., 


_  Magnetomotive  Force 
Reluctance 


In  this  respect  electric  current  and  magnetic  lines  are 
similar.  However,  while  electric  circuits,  in  the  main,  ex- 
ist in  media  of  zero  electric  conductivity,  and  therefore 
permit  of  accurate  calculations,  there  being  no  appreciable 
leakage,  magnetic  circuits  must  be  situated  in  media  which 


MAGNETIC    LAWS   AND    FACTS. 


have  permeabilities  of  at  least  unity.  In  the  latter  case 
much  leakage  is  present,  and  precise  calculations  are  out  of 
the  question.  In  the  designing  of  dynamo  electric  ma- 
chinery, however,  one  or  more  paths  of  low  reluctance  are 
presented  to  the  magnetizing  force,  and  these  are  pro- 
tected by  being  so  shaped  that  leakage  paths  offer  a  com- 
paratively high  reluctance. 

26.     Hysteresis.  —  If  a  piece  of  iron  become  magnetized, 
and  the  magnetizing  force  be  then  removed,  the  iron  does 


Magnetlztng 


Fig.  xi. 

not  become  completely  demagnetized.  A  certain  magnet- 
izing force  in  the  opposite  direction  must  be  used  to  bring 
it  to  a  neutral  state.  This  phenomenon  has  been  termed 
hysteresis.  Because  of  hysteresis  a  (&— 3C  curve  taken  with 
continuously  increasing  values  of  3C  to  the  maximum  and 


28  DYNAMO   ELECTRIC  MACHINERY. 

then  with  continuously  decreasing  values  of  3C  to  a  negative 
maximum,  and  so  on,  will  assume  the  shape  shown  in  Fig. 
1  1  .  The  distance  O  A  represents  the  coercivity,  that  is, 
the  magnetizing  force  necessary  to  bring  the  iron  from  a 
magnetic  to  a  neutral  state.  The  distance  O  C  represents 
the  retentivity,  that  is,  the  amount  of  magnetic  induction 
left  in  the  iron  after  the  magetizing  force  has  been  removed. 
The  area  inclosed  by  the  curve  represents  the  energy  lost 
in  carrying  the  iron  through  one  cycle,  i.e.,  from  a  maximum 
magnetization  to  a  maximum  in  the  opposite  direction  and 
back  to  the  orginal  condition. 

For  suppose  the  magnetization  to  be  due  to  a  current  7 
flowing  in  a  solenoid  of  n  turns.  If,  in  a  short  interval  of 
time  dt,  a  change  of  d$  be  made  in  the  flux  which  is  linked 
with  the  solenoid,  then  this  change  will  induce  an  E.M.F. 
in  the  solenoid  which  during  the  interval  of  time  dt  will  be 
equal  to 

nd<$> 

E  =  —  £-  volts. 
ioV/ 

During  this  time  work  must  be  performed  to  maintain  this 
current  /,  and  its  magnitude  is 


io 

for  Idt  represents  the  quantity  of  electricity  which  is  trans- 
ferred from  one  point  to  another,  between  which  there  ex- 
ists a  difference  of  potential  E.  Now  $  =  A($>  (§22)  and 

I  O  TP/ 

hence  d$  =  Ad<$>.  Furthermore,  nl  =  -  —(§21).  Hence 
the  work  during  the  time  dt  is 

Eldt  =  —  7—  5C  <i<$>  joules. 

I0747T 


MAGNETIC   LAWS   AND   FACTS.  29 

Supposing  the  magnetizing  force  to  vary  cyclically,  taking 
T  seconds  to  make  one  cycle,  then  the  work  per  cycle  is 


Al     r 
EIT=  —  _—  /  3C</(B  joules. 

10  VJ     ._  (R      ' 
UJ/Hax 

If  the  number  of  cycles  completed  in  one  second  be/,  then 
f  =  —  ,  and  the  work  in  joules  per  second,  that  is,  the  power 
in  watts,  equals 


f    A    J          f*     1       ^IIHl  K  >7        A  /*     I     ^/wax 

^/-  ^—-  /   3C^/(B    =— 79    / volume  /  X./&. 
io'47rj    _(B;))aj  I0  J   -(B,^ 

The  integral  expression  is  evidently  the  area  contained  by 
the  hysteresis  loop. 

27.  Steinmetz's  Law The  value  of  the  integral  ex- 
pression is  dependent  upon  (&„,„„  upon  the  retentivity  of 
the  kind  of  iron,  and  upon  its  coercivity.  Steinmetz  has 
shown  that  for  all  practical  purposes  the  value  of  the  inte- 
gral may  be  expressed  by  the  empirical  formula 

'+«,„„. 


where  77  is  a  constant   depending  upon  the  kind  of  iron. 
Its  value  is  given  in  the  following  table  :  — 

HYSTERETIC    CONSTANTS. 

Best  soft  iron  or  steel  sheets o.ooi 

Good  soft  iron  sheets 0.002 

Ordinary  soft  iron 0.003 

Soft  annealed  cast  steel 0.008 

Cast  steel 0.012 

Cast  iron 0.016 

Hard  cast  steel 0.025 


30  DYNAMO   ELECTRIC  MACHINERY. 

The  hysteretic  constant,  if  at  first  small,  grows  with  age. 
Its  increase  can  be  hastened  by  continued  heating.  The 
increase  may  amount  to  200  per  cent.  Annealing,  while  it 
increases  the  permeability,  also  increases  the  hysteretic 
constant,  if  it  be  originally  very  small. 

The  magnitude  of  the  hysteretic  constant  is  largely  de- 
pendent upon  the  mechanical  structure  of  the  iron.  To 
attain  the  smallest  value  the  iron  should  not  be  of  homoge- 
neous structure,  but  should  have  a  greater  density  in  the 
direction  perpendicular  to  the  direction  of  flux. 


ARMATURES.  31 


CHAPTER  III. 

ARMATURES. 

28.  Dynamos Dynamos  may  be  defined  as  machines 

to  convert    mechanical    energy  into    electrical    energy    by 
means  of  the  principle  of  electromagnetic  induction.     In 
all  commercial  machines  the  mechanical  energy  is  supplied 
in  the  form  of  rotation,  and  the  electrical  energy  is  deliv- 
ered either  as  "  direct  current"  or  "alternating  current." 
These  machines  are  also  frequently  called  generators. 

29.  Principle  of  the  Action  of  a  Dynamo.  —  If  a  loop  of 
wire  be  revolved  in  a  magnetic  field  about  an  axis  perpen- 
dicular to  the  lines  of   force,  as  in  Fig.  1 2,  then  each  side 
(but  not  the  ends)  of  the  loop  is  a  conductor  moving  across 
the  lines  of  a  magnetic   field,  and   as  such   will  have  an 
E.M.F.  induced  in  it.     Since  the  motion  of  one  conductor 
is  up  while  that  of  the  other  is  down,  the  directions  of  the 
induced  E.M.F. 's  in  the  two  sides  will  be  opposite  to  each 
other,  and  since  they  are  on  opposite  sides  of  a  loop,  the 
pressure  will  be  cumulative  ;  i.  e.,  instead  of  neutralizing 
each  other,  the  two  pressures  will  be  added  to  each  other. 
If  now  the  two  ends  of  the  wire  from  which  the  loop  is 
made  be  respectively  connected  with  slip  rings,  and  a  cir- 
cuit be  completed  through  contacts  sliding  on  them,  a  cur- 
rent will  flow.     When  the  loop,  in  its  revolution,  reaches  a 
position  (as  illustrated  in  Fig.  1 2)  such  that  the  conductor 


DYNAMO  ELECTRIC   MACHINERY. 


that  was  previously  moving  upward  begins  to  move  down- 
ward, then  the  direction  of  the  induced  E.M.F.  will  be 
changed  in  both  sides  of  the  loop,  and  the  direction  of  the 


Fig.  12. 

current  through  the  circuit  will  be  changed.  For  each 
complete  revolution  the  current  changes  direction  twice. 
It  is  an  alternating  current,  and  the  supposed  machine  is 
an  alternating  current  dynamo,  or  simply  an  alternator. 

30.  The  Principle  of  the 
Commutator A  commu- 
tator is  used  on  the  shaft  of 
a  machine  when  it  is  de- 
sired to  get  a  direct  or  rec- 
tified current.  For  the 
single  loop  in  the  above 
case,  the  commutator  (Fig. 
13)  would  consist  of  two  similar  cylindrical  parts  of  metal, 
insulated  from  each  other,  and  affording  sliding  contact  for 


Fig.   13. 


ARMATURES. 


33 


two  brushes.  One  end  of  the  wire  of  the  loop  is  attached 
to  one  piece  of  the  commutator,  and  the  other  to  the  other. 
The  brushes  are  so  placed  that  at  the  instant  the  in- 
duced E.M.F.  in  the  loop  changes  its  direction,  the  brushes 
slide  across  from  one  segment  of  the  commutator  to  the 
other,  and  thus  the  current,  while  reversed  in  the  loop,  is 


Fig.  14. 

left  flowing  in  the  same  direction  in  the  outside  circuit. 
If  the  loop  were  wound  double  before  the  ends  were  at- 
tached to  the  commutator  segments,  and  if  the  speed  of 
revolution  and  the  strength  of  the  magnetic  field  were  both 
maintained  constant,  twice  the  E.M.F.  would  be  produced, 
but  no  more  commutator  segments  would  be  necessary 
(Fig.  14). 

In  the  above  cases  at  the  instants  of  commutation  there 
would  be  no  E.M.F.  produced,  and  hence  the  current  would 
fall  to  zero  twice  every  revolution.  If  two  coils  were  placed 
90°  apart,  one  or  the  other  would  always  be  cutting  lines 
of  force.  Hence  at  no  time  could  the  pressure  be  zero. 


34 


DYNAMO  ELECTRIC  MACHINERY. 


To  satisfactorily  collect  current  from  this  arrangement  re- 
quires four  commutator  segments  and  a  system  of  connec- 
tions similar  to  that  shown 
in  Fig.  15.  In  this  case 
the  E.M.F.  would  fluctu- 
ate, but  not  so  badly  as  in 
the  previous  case.  If  we 
increase  the  number  of 
loops,  and  correspondingly 
increase  the  number  of 
commutator  segments,  we 
decrease  the  fluctuation 
of  the  E.M.F.  until  it  be- 
comes practically  constant.  In  a  bipolar  machine  with  12 
commutator  segments  the  fluctuation  is  1.7  per  cent  of  the 
total  E.M.F. 


Fig,  15. 


31.   The  Armature In  a  dynamo,  the  loops  of  wire  in 

which  E.M.F.  is  induced  by  movement  in  a  magnetic  field, 
together  with  the  iron  core  that  sustains  them,  with  the 
necessary  insulation,  and  with  the  parts  connected  imme- 
diately thereto,  constitute  the  armature  of  a  dynamo.  The 
conductors  in  which  the 
E.M.F.  is  generated  are 
called  the  inductors.  An 
armature  in  which  both 
sides  of  the  loop  of  wire 
cut  lines  of  force,  as  in 
the  cases  just  described, 

11     i  T-L 

is  called  a  Drum  Arma- 
ture.    A  kind  of  armature  less  generally  used  is  the  Ring 
Armature,  illustrated  diagrammatically  in   Fig.   16.      Here 


ARMATURES. 


35 


the  lines  of  force  emanating  from  the  N.  pole  of  the  field 
magnets  flow  through  the  iron  core  of  the  ring,  and  very 
few  across  the  air  space  inside  the  ring.  Hence  when 
wires  are  wound  on  the  ring,  and  the  whole  is  revolved 
about  an  axis  perpendicular  to  the  plane  of  the  ring,  only 
the  wires  on  the  outside  face  of  the  ring  cut  lines  of  force, 
those  on  the  inside  serving  only  to  complete  the  electrical 
circuit.  So  a  smaller  portion  of  the  wire  on  a  ring  arma- 
ture is  in  action  than  on  a  drum  armature. 

A  drum  armature  of  large  diameter  and  of  short  length 
in  the  axial  direction  has  more  wire  exposed  on  its  ends 
than  on  its  periphery.  The  pole  pieces  are  sometimes 
placed  at  the  ends,  and  the  armature  is  then  called  a  Disk 
Armature.  This  type  is  seldom  used  in  this  country. 

32.  The  Field  Magnets.  —  Almost  all  dynamos  have 
their  magnetic  fields  produced  by  electro-magnets.  These1 


Fig.  17. 


are  called  the  field  magnets.     In  small  machines  these  are 
usually  bipolar,  i.e.,  having  one  N.  and  one  S.  pole,  with 


36  DYNAMO   ELECTRIC  MACHINERY. 

the  armature  revolving  between.  In  large  machines  it  is 
usual  to  use  multipolar  field  magnets,  in  which  any  even 
number  of  poles  alternately  N.  and  S.  are  arranged  in  a 
circle  with  their  faces  concentric  with  the  armature. 

Bipolar  machines  are  made  in  many  forms,  a  few  of 
which  are  shown  in  Fig.  17. 

The  magnetizing  coils  may  be  on  both  legs  of  the  mag- 
net, on  one  leg,  or  on  the  yoke  which  connects  the  two  legs. 
In  the  double  horse-shoe  type  there  are  four  windings,  one 
on  each  of  the  four  legs.  Such  a  field  is  sometimes  said 
to  be  of  the  consequent  pole  type. 

33.  Capacity  of  a  Dynamo.  —  By  §  13,  in  a  bipolar 
machine  the  average  pressure  between  brushes  equals  the 
product  of  the  number  of  lines  cut  into  the  number  of  in- 
ductors cutting  them,  divided  by  the  time  in  seconds  of 
one  revolution.  Since  each  line  is  cut  twice  in  one  revolu- 
tion by  each  conductor,  the  formula  for  the  E.M.F.  pro- 
duced by  the  machine  is 


^  --  8 

60  icr 

where  V  is  the  number  of  revolutions  per  minute,  <£  the 
total  flux  through  the  loops,  and  ^  the  number  of  inductors. 
In  drum  armatures  5  =  twice  the  number  of  loops  ;  in  ring 
armatures  5  =  the  number  of  loops. 

The  capacity  of  a  machine  is  measured  by  the  watts  it 
can  send  out,  hence  the  capacity  varies  as  EL  It  is  seen 
from  the  foregoing  formula  that  the  £of.  any  machine  may 
be  increased  by  increasing  either  V,  <£,  or  5. 

The  value  of  Fis  limited,  (i)  by  considerations  of  me- 
chanical safety  and  economy,  and  (2)  by  the  desirability,  in 
the  case  of  a  dynamo,  of  directly  connecting  it  to  the  steam 


ARMATURES.  37 

engine  or  other  prime  mover,  and  in  the  case  of  a  motor 
the  connection  of  it  to  the  machine  it  operates.  The  speed 
of  small  machines  is  greater  than  that  of  larger  ones ;  but 
the  peripheral  velocity,  that  is,  the  velocity  of  a  point  on 
the  exterior  of  the  armature,  for  all  sizes,  lies  between  25 
and  100  feet  per  second  on  belt-driven  machines,  and  be- 
tween 25  and  50  feet  per  second  on  direct  connected 
machines.  On  large  (say  2,000  K.W.)  multipolar  machines, 
having  great  diameter  of  armature,  these  values  are  often 
exceeded. 

The  value  of  <£  depends  upon  the  size  of  the  machine, 
and  the  permeability  of  the  metal  of  its  frame.  To  get  a 
large  and  economical  (B  the  metal  parts  of  the  field  magnets 
are  designed  to  have  a  very  low  magnetic  reluctance.  The 
air-gap  between  the  pole  pieces  and  the  armature,  and  the 
space  occupied  by  the  revolving  inductors,  are  each  made 
small.  The  armature  inductors  are  wound  upon  an  iron 
core  of  low  magnetic  reluctance.  These  cores  are  fre- 
quently slotted  and  the  windings  laid  in  the  slots.  Besides 
reducing,  to  a  certain  extent,  the  magnetic  reluctance  by 
this  construction,  a  good  mechanical  means  is  furnished 
for  driving  and  protecting  the  inductors.  Wires  wound  on 
the  exterior  of  a  plain  cylinder,  or  smooth  core,  under  the 
influence  of  high  speeds  and  the  "magnetic  drag"  which 
they  experience  have  a  serious  tendency  to  rub  one  an- 
other, and  chafe  the  insulation  to  its  final  destruction. 
The  armatures  having  slotted  cores,  which  are  also  called 
toothed  core  armatures,  are  to  be  recommended  for  gene- 
rators that  will  be  obliged  to  work  under  wide  variations  of 
load.  They  cost  more  to  build  than  smooth-core  arma- 
tures. 

The  numbers  of  inductors  5  on  an  armature  can  be  in- 


38  DYNAMO  ELECTRIC  MACHINERY. 

creased  by  decreasing  the  size  of  the  wire.  Sufficient 
cross-section  must,  however,  be  left  in  the  inductors  to 
carry  the  maximum  current  of  the  machine  without  causing 
a  heating  of  the  armature  to  such  a  point  as  to  endanger 
the  insulation.  Good  practice  calls  for  from  400  to  800 
circular  mils  cross-section  of  armature  conductor  per  am- 
pere. The  smaller  values  are  for  intermittently  acting 
machines  —  elevator  motors  for  example.  The  larger 
values  are  for  machines  that  run  continuously,  such  as 
central-station  generators. 

34.    Eddy  or  Foucault  Currents  in  Armature  Cores.  — 

It  is  evident  that  an  imaginary  axial  lamina  of  the  iron  core 
of  an  armature  is  a  conductor  moving  in  a  field,  and  there- 
fore has  in  it  an  induced  E.M.F.  Since  this  lamina  in  it- 


self forms  a  closed  circuit,  currents,  called  Foucault  or  eddy 
currents,  will  flow  in  it,  Fig.  18,  and  their  energy  will 
appear  in  the  form  of  heat,  which  will  produce  an  undue 
elevation  of  temperature  of  the  armature.  To  avoid  this 
the  iron  of  the  core  is  laminated  at  right  angles  to  the  axis 
of  revolution,  and  the  laminae  are  insulated  from  one 


ARMATURES.  39 

another.  The  heating  due  to  eddy  currents  is  proportional 
to  the  square  of  the  thickness  of  the  disks  or  laminae. 
Commercial  and  mechanical  reasons  limit  the  decrease  of 
thickness.  In  good  practice  the  thickness  of  armature 
disks  varies  from  .01"  to  .06." 

For  insulation  between  the  disks  reliance  is  usually 
placed  on  the  iron  oxid  that  forms  on  them  during  their 
manufacture.  Generally  every  six  disks  or  so  a  further 
insulation  is  interposed  by  the  use  of  shellac,  japan,  or 
paper.  Milling  slots  in  laminated  armature  cores  after  set- 
ting up  causes  burrs.  These  bridge  the  insulation  between 
the  disks,  and  militate  against  the  advantages  sought  after 
by  lamination.  For  small  armatures  the  disks  are  punched 
whole  from  sheet -iron,  with  the  teeth  and  holes  for  the 
shaft.  These  punchings  are  assembled  on  the  shaft,  and 
held  in  place  by  brass  collars  set  down  on  either  side  of  the 
pile  by  nuts  on  the  shaft  or  by  similar  devices.  In  large 
machines,  parts  or  segments  of  the  whole  periphery  are 
punched  separately,  and  these  are  assembled  with  joints 
staggered.  These  large  laminae  are  not  directly  attached 
to  the  shaft,  but  are  mounted  upon  a  spider,  which  in  turn 
is  connected  with  the  shaft.  A  complete  spider  and  core 
is  shown  in  Fig.  19. 

In  large  armatures  it  is  usual  to  make  ducts  or  venti- 
lating passages  in  the  core  by  occasionally  separating  the 
disks  by  the  interposition  of  blocks  of  insulating  material. 
Such  ventilation  carries  off  the  heat,  and  lessens  the  rise  of 
temperature  of  the  armature  when  in  operation. 

35.  Rating  of  Machines.  —  The  American  Institute  of 
Electrical  Engineers  recommends  that  all  electrical  and 
mechanical  power  be  expressed,  unless  otherwise  specified, 


DYNAMO   ELECTRIC   MACHINERY. 


Fig.  19. 


ARMATURES.  41 

in  kilowatts  ;  that  the  full-load  current  of  an  electric  gene- 
rator be  that  current  which,  with  the  rated  full-load  volt- 
age, gives  the  rated  kilowatts  ;  that  all  guaranties  on  heat- 
ing, regulation,  and  sparking  should  apply  to  the  rated 
load,  except  where  expressly  specified  otherwise ;  that 
direct  current  generators  should  be  able  to  stand  an  over- 
load of  25  per  cent  for  one-half  hour  without  an  increase 
of  temperature  elevation  exceeding  15°  C.  above  that 
specified  for  full  load ;  and  that  direct  current  motors 
should,  in  addition,  be  able  to  stand  an  overload  of  50  per 
cent  for  one  minute. 

Concerning  the  normal  permissible  elevation  of  tempera- 
ture the  following  statements  are  taken  from  articles  25  to 
3 1  of  the  Institute's  Standardization  Report :  — 

"  Under  regular  service  conditions,  the  temperature  of 
electrical  machinery  should  never  be  allowed  to  remain  at 
a  point  at  which  permanent  deterioration  of  its  insulating 
material  takes  place. 

"  The  rise  of  temperature  should  be  referred  to  the  stan- 
dard conditions  of  a  room  temperature  of  25°  C.,  a  baro- 
metric pressure  of  760  mm.  and  normal  conditions  of 
ventilation  ;  that  is,  the  apparatus  under  test  should  neither 
be  exposed  to  draught  nor  inclosed,  except  where  ex- 
pressly specified. 

"  If  the  room  temperature  during  the  test  differs  from 
25°  C.,  the  observed  rise  of  temperature  should  be  cor- 
rected by  £  per  cent  for  each  degree  C.  Thus,  with  a 
room  temperature  of  35°  C.,  the  observed  rise  of  tem- 
perature has  to  be  decreased  by  5  per  cent,  and  with 
a  room  temperature  of  15°  C.,  the  observed  rise  of  tem- 
perature has  to  the  increased  by  5  per  cent.  The 
thermometer  indicating  the  room  temperature  should 


42  DYNAMO  ELECTRIC  MACHINERY. 

be  screened  from  thermal  radiation  emitted  by  heated 
bodies,  or  from  draughts  of  air.  When  it  is  impracti- 
cable to  secure  normal  conditions  of  ventilation  on  ac- 
count of  an  adjacent  engine,  or  other  sources  of  heat,  the 
thermometer  for  measuring  the  air  temperature  should  be 
placed  so  as  fairly  to  indicate  the  temperature  which  the 
machine  would  have  if  it  were  idle,  in  order  that  the  rise  of 
temperature  determined  shall  be  that  caused  by  the  opera- 
tion of  the  machine. 

"  The  temperature  should  be  measured  after  a  run  of 
sufficient  duration  to  reach  practical  constancy.  This  is 
usually  from  6  to  1 8  hours,  according  to  the  size  and  con- 
struction of  the  apparatus.  It  is  permissible,  however,  to 
shorten  the  time  of  the  test  by  running  a  lesser  time  on  an 
overload  in  current  and  voltage,  then  reducing  the  load  to 
normal,  and  maintaining  it  thus  until  the  temperature  has 
become  constant. 

"  In  apparatus  intended  for  intermittent  service,  as  rail- 
way motors,  starting  rheostats,  etc.,  the  rise  of  temperature 
should  be  measured  after  a  shorter  time,  depending  upon 
the  nature  of  the  service,  and  should  be  specified. 

"  In  apparatus  built  for  conditions  of  limited  space,  as 
railway  motors,  a  higher  rise  of  temperature  must  be 
allowed. 

"  In  electrical  conductors,  the  rise  of  temperature  should 
be  determined  by  their  increase  of  resistance.  For  this 
purpose  the  resistance  maybe  measured  either  by  galva- 
nometer test  or  by  drop-of -potential  method.  A  temperature 
coefficient  of  0.4  per  cent  per  degree  C.  may  be  assumed 
for  copper.  Temperature  elevations  measured  in  this  way 
are  usually  in  excess  of  temperature  elevations  measured 
by  thermometers. 


ARMATURES. 


"  It  is  recommended  that  the  following  maximum  values 
of  temperature  elevation  should  not  be  exceeded :  — 

COMMUTATING    MACHINES. 

Field  and  armature  by  resistance,  50°  C. 

Commutator  and  brushes  by  thermometer,  55°  C. 

Bearings  and  other  parts  of  machine,  by  thermometer,  40°  C. 

"  Where  a  thermometer,  applied  to  a  coil  or  winding,  in- 
dicates a  higher  temperature  elevation  than  that  shown  by 
resistance  measurement,  the  thermometer  indication  should 
be  accepted.  In  using  the  thermometer,  care  should  be 
taken  so  to  protect  its  bulb  as  to  prevent  radiation  from  it, 
and,  at  the  same  time,  not  to  interfere  seriously  with  the 
normal  radiation  from  the  part  to  which  it  is  applied. 

"  In  the  case  of  apparatus  intended  for  intermittent  ser- 
vice, the  temperature  elevation,  which  is  attained  at  the  end 

of  the  period  corresponding  to 
the  term  of  full  load,  should 
not  exceed  50°  C.  by  resistance 
in  electric  circuits.  In  the  case 
of  railway,  crane,  and  elevator 
motors,  the  conditions  of  ser- 
vice are  necessarily  so  varied 
that  no  specific  period  corre- 
sponding to  the  full-load  term 
can  be  stated." 

The  manner  in  which  temper- 
ature elevation  is  affected  by  size  of  load  and  duration  of 
full  load  is  shown  in  Figs.  20  and  21.  The  temperature 
of  stationary  surfaces  rises  about  80°  when  radiating  one 
watt  per  square  inch.  The  rise  is  but  15°  to  20°  when  the 


300 


200 


100 


8 

1    1    1 

1 

g     RJSE  IN  TEMPERATURE  CURV 
"p7            800  K.   W.       SIZE  280 
g-                   DIRECT  DYNAMO 
"    RUN  LONG   ENOUGH   AT  EACK  LOAD 
•«-     ATTAIN  A  CONSTANT  TEMPERATMI 
U-     CROCKER-WHEELER  ELECTRIC  CC 
0                        AMPERE,  N.   J. 

E 

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TO 

/ 

., 

-/- 

o" 

/ 

/ 

1 

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V    —  ' 
z 

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Si 

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j 

OA 

TE 

RM! 

U 

fl 

LL 

LO* 

n 

422 

1 

Fig.  20. 


44 


DYNAMO  ELECTRIC   MACHINERY. 


surface  is  rotating  at    3,000  feet   per   minute   in    such   a 
manner  as  the  surface  of  an  armature  rotates,  and  amounts 


<u 
110 
50 
.40 
30 
20 
10 
0 

| 

,.  ' 

— 

_— 

S 

tff 

-^ 

| 

^ 

X 

£ 

x 

TEM 
300 
D 

UNO 

CROCKEF 

PERATURE  CL 
K.  W.      SIZE 
IRECT  DYNAIV 

JRVE 
280 
0 

OAD 

RICCO.  , 

0 

/ 



S 

/ 

/ 

-WHEELER  ELECT 
AMPERE,  N.  J. 



| 

/ 

I 

P 

5 

1 

TIME 

N 

HO 

URS 

421 

0      .1 


1567 
Fig.  21. 


8      9      10 


to  but  10°  to  12°  at  a  speed  of  6,500  feet  per  minute. 
Within  limits,  the  ratio  of  rise  of  temperature  to  radiation 
per  unit  surface  is  linear. 

36.   Definitions   Concerning    Armature    Windings.  —  In 

some  dynamos  the  inductors  and  commutator  segments  are 
not  all  electrically  connected  with  each  other.  In  such 
cases  the  winding  is  called  an  open-coil  winding.  This 
definition  must  not  be  made  to  include  the  double  or  mul- 
tiple windings  to  be  described  later,  where  two  or  more 
closed-coil  windings  on  the  same  core  are  not  electrically 
connected  to  one  another.  Fig.  22  shows  a  primitive  open- 
coil  winding.  In  this  type  only  those  inductors  on  whose 
commutator  bars  the  brushes  may  for  the  moment  be  rest- 
ing are  in  series  with  the  external  circuit.  All  the  other 
inductors  are  cut  out  and  idle. 

Open-coil  windings  are  used  chiefly  on  arc-lighting  dyna- 
mos, and  will  be  further  discussed  in  a  following   chap- 


ARMATURES. 


45 


ter  devoted  to  such  machines.  Closed-coil  windings  are 
much  more  generally  used.  In  this  case  all  the  inductors 
are  engaged  all  the  time,  save  when  short  circuited  at  com- 
mutation, in  adding  E.M.F.  to  the  circuit.  Although  there 
are  many  kinds  of  closed-coil  windings,  they  are  all  alike 
in  that  the  inductors  form  one  or  more  endless  circuits 
completely  around  the  armature  core. 

Before  showing  some  of  the  many  types  of  closed-coil 
winding  it  will  be  well  to  define  some  of  the  terms  used. 


Fig.  22. 

By  inductor  is  meant  that  part  of  the  winding  conductor 
which  lies  on  the  face  of  the  armature  that  sweeps  past 
the  pole  pieces,  and  is  that  part  of  the  conductor  in  which 
E.M.F.  is  induced.  In  the  following  descriptions  when 
one  inductor  is  mentioned  there  may  be  in  reality  a  num- 
ber of  wires  ;  and,  again,  a  loop  said  to  be  formed  by  two 
inductors  may  be  a  loop  of  many  turns,  but  the  connec- 
tions and  placing  would  be  the  same  as  if  actually  there 


46  DYNAMO  ELECTRIC   MACHINERY. 

were  only  one  inductor.  It  simplifies  the  diagrams  to 
treat  the  subject  in  this  manner. 

That  part  of  an  armature  winding  which  is  electrically 
connected  directly  between  two  consecutive  commutator 
segments  is  called  a  coil. 

A  two-circuit  winding  is  one  in  which  the  current,  on 
entering  the  armature  at  one  brush,  finds  two  paths  by 
which  it  reaches  the  other  brush.  Since  a  closed-coil 
winding  is  endless,  there  must  invariably  be  two  paths 
when  two  brushes  are  used. 

K  four-circuit  or  multi-circuit  winding  is  one  in  which 
the  current  finds  four  or  more  paths  through  the  arma- 
ture. There  are  at  least  two  circuits  for  every  pair  of 
brushes  used  in  collecting  the  current,  unless  the  commu- 
tator bars  are  cross-connected,  as  in  Fig.  25. 

If  a  winding  is  so  arranged  that  one  commutator  bar 
under  a  brush  carries  all  the  current  from  one  side  of  the 
armature  to  that  brush,  then  the  winding  is  said  to  be 
single.  If,  however,  the  windings  are  so  arranged  that 
two  or  more  bars  convey  this  current  to  the  brush  at  once, 
or  if  the  current  is  commutated  at  two  or  more  points  on 
the  contact  surface  of  the  brush,  then  the  winding  is  said 
to  be  double  or  multiple.  Triple  and  quadruple  windings 
are  not  infrequent  on  machines  which  carry  very  heavy 
currents. 

A  singly-re-entrant  winding  is  one  in  which,  by  successive 
angular  advances,  all  the  coils  have  been  laid  when  an 
advance  of  360°  has  been  made.  To  be  doubly-re-entrant 
wound  the  angular  advance  between  successive  coils,  in  the 
order  of  their  winding,  is  doubled  ;  and  the  whole  winding 
is  not  complete  until  the  armature  has  been  gone  around, 
angularly,  twice,  i.e.,  through  an  advance  of  720°.  On  the 


ARMATURES.  47 

second  time  around  the  coils  fill  up  the  interstices  left  by 
the  doubled  pitch  of  the  first  round.  Triply  and  quadruply 
re-entrant  windings  are  used.  In  these  the  circuit  passes 
around  the  armature  three  or  four  times.  Any  closed-coil 
winding,  single  or  multiple,  may  be  singly  or  multiply  re- 
entrant, the  re-entrancy  being  reckoned  as  great  as  that  of 
any  single  winding  on  the  armature. 

The  two  principal  types  of  closed-coil  armatures  are 
the  gramme  or  ring  armature,  and  the  drum. 

37.  Ring-Armature  Windings.  —  As  the  name  implies, 
the  ring-armature  core  consists  of  an  annular  ring  around 
which  the  armature  conductors  are  wound  in  a  continuous 
spiral,  or  two  or  more  separate  but  interleaved  spirals  in 
multiple  windings.  These  are  tapped  off  at  equal  inter- 
vals to  the  commutator  bars.  In  ring  armatures  there  is 
but  one  inductor  per  loop  of  wire,  the  return  being  on  the 
inside  of  the  ring  where  there  is  no  magnetic  flux.  This 
winding,  though  less  generally  used  than  the  drum  winding 
is  simpler  and  much  more  easily  illustrated,  and  will  be 
treated  first. 


Fig.  24. 

Fig.  23  shows  the  simplest  of  all  dynamo  armature 
windings.  It  is  a  bipolar,  singly-re-entrant,  two-circuit, 
single  winding. 

Fig.  24  shows  a  four-pole,  four-circuit,  singly-re-entrant, 


48 


DYNAMO  ELECTRIC  MACHINERY. 


single  winding.  The  number  of  coils  should  be  a  multiple 
of  the  number  of  poles  to  electrically  preserve  a  balance  in 
the  four  branches  or  circuits. 

Fig.  25  is  the  same  as  Fig.  24  save  that  the  commutator 
bars  are  cross-connected.  The  current  that  would  flow 
out  of  two  brushes  in  the  previous  case,  now  flows  out  of 
one  brush.  This  form  is  seldom  used,  since  it  reduces  by 
half  the  brush  contact  surface,  and  thus  doubles  the  heat 


Fig.  25. 


loss  in  the  transition  of  the  current  from  the  commutator 
to  the  brush. 

Fig.  26  shows  a  four-pole  four-circuit  singly-re-entrant, 
single  winding,  where  only  half  as  many  bars  are  used  as 
there  are  coils.  A  disadvantage  is  that  coils  of  considerable 
difference  of  pressure  are  adjacent,  thus  increasing  the 
difficulty  of  properly  insulating  them.  Ordinarily,  if  it  be 
desired  to  halve  the  'number  of  bars,  it  is  better  to  unite 
two  adjacent  coils  in  series,  and  treat  them  as  one.  But 
if  the  magnetic  distribution  be  uniform,  this  method  of 
connecting  two  coils  that  are  in  different  parts  of  the  field 


ARMATURES. 


49 


in  series  averages  up  the  inequalities  and  facilitates  spark- 
less  commutation. 

Fig.  27  shows  a  bipolar,  two-circuit,  singly-re-entrant, 
double  winding.  The  advantages  of  the  double  winding 
are :  the  current  is  commutated 
at  two  points  of  the  bearing-sur- 
face of  the  brush,  and  therefore  is 
only  half  as  heavy  at  any  one  point 
as  when  only  a  single  winding  is 
used  ;  and  the  successive  bars  of 
one  winding  are  separated  by  the 
width  of  one  bar  plus  two  insula- 
tions, thus  making  the  short  cir- 
cuiting of  a  coil  by  dirt,  arc,  or 
injury  very  unlikely. 

In  multipolar  windings  a  distinc- 
tion is  made  between  the  "  short- 
connection  "   and  the   "  long-connection  "  types.       In  the 
short-connection  type  coils  in  adjacent  fields  are  connected 

in  series,  while  in  the  long-con- 
nection type  coils  twice  as  far 
apart  are  connected  together. 

Fig.  28  shows  a  long-connec- 
tion, two-circuit,  four-pole  single 
winding.  Here  only  slight  dif- 
ferences of  potential  exist  be- 
tween contiguous  coils. 

Fig.  29  represents  a  ten-pole, 
long-connection,  two-circuit,  sin- 
gle winding.  In  these  long-con- 
nection types,  which  are  all  more  or  less  highly  re-entrant, 
small  mention  is  made  of  the  re-entrancy.  Strictly  accord- 


Fig.  27. 


50  DYNAMO  ELECTRIC  MACHINERY. 

ing  to  definition,  the  winding  in  Fig.  29  is  re-entrant  nine 

times. 

Fig.  30  is  a  four- 
pole,  short -connection, 
two-circuit,  single  wind- 
ing. Besides  the  com- 
plication of  the  wind- 
ings, this  form  as  well 
as  all  other  short-con- 
nection windings,  is 
open  to  the  objection 
that  the  contiguous 
coils  have,  periodically, 
the  full  E.M.F.  of  the 
machine  between  them, 

making  heavy   insulation   necessary. 

Fig.    31    gives    a    four-pole,    two-circuit    double-wound 


Fig.  30. 


Fig.  31. 


armature,  and  Fig.   32  gives  a  similar  winding  for  a  six- 
pole  machine. 


ARMATURES.  51 

38.  Drum  Armature  Windings.  —  Windings  for  drum 
armatures  are  more  varied  and  more  complex  than  those 
for  ring  armatures,  and  are  much  harder  to  portray  dia- 
grammatically.  But  few  will  be  shown. 

The  most  simple  of  these  windings  is  shown  in  Fig.  33. 
The  diagram  shows  the  drum  and  inductors  in  section, 
with  the  connections  of  the  commutator  end  in  full  lines 
and  those  of  the  back  (pulley)  end  in  dotted  lines.  Those 
inductors  marked  with  a  4-  are  supposed  to  carry  a  current 
in  the  direction  from  the  observer  into  the  paper,  and 


Fig.  32.  Fig.  33. 

those  marked  with  a  are  supposed  to  carry  a  current  from 
the  page  to  the  observer.  Those  not  marked  are  parts  of 
coils  short-circuited  by  the  brushes.  This  winding  was 
devised  by  von  Hefner-Alteneck,  and  may  be  used  on  any 
bipolar  armature  having  half  as  many  commutator  bars  as 
slots,  or,  if  it  be  smooth  core,  as  many  bars  as  coils.  If  n 
be  the  number  of  bars  and  2n  the  number  of  slots,  then 
the  wire  is  started  at  bar  I,  passed  back  through  slot  I, 
across  the  pulley  end  to  slot  n  (or  sometimes  n  ±  2,  in  the 


52  DYNAMO  ELECTRIC  MACHINERY. 

figure  n  =  8).  It  is  then  brought  forward  through  slot  n, 
and  attached  to  bar  2.  From  bar  2  it  passes  back  through 
slot  3,  across  the  back  end  and  forward  through  slot  n  +  2 
and  connects  to  bar  3.  Thus  passing  back  through  the 
odd-numbered  slots  and  forward  through  the  even-num- 
bered slots,  n  coils  can  be  made  to  fill  the  2;/  slots  and 
each  can  be  attached  to  its  own  commutator  bars. 

Fig.  34  is  very  similar  to  the  last,  save  that  the  wires 
are  laid  two  layers  deep,  thus  allowing  the  conductor  that 

passed  through  slot  i  to  return 
through  slot  n  +  I  which  is 
diametrically  opposite.  Both 
this  winding  and  the  last  are 
classed  as  two-circuit  single 
windings. 

In  a  bipolar  machine  a  chord- 
wound  drum  armature  is  one  in 
which  the  two  inductors  of  one 
loop  are  appreciably  less  than 
1 80°  apart,  so  that  the  wire  at 
Fig-  34-  the  back  end  is  a  chord  rather 

than  a  diameter  of  the  circle  of  the  drum.  The  advan- 
tages of  this  winding  are  that,  on  a  given  drum,  it  de- 
creases the  total  length  of  wire  necessary  to  give  a  definite 
number  of  inductors,  and  that  it  reduces  the  bunching  and 
overlapping  of  the  wires  at  the  pulley  end  of  the  drum. 
The  disadvantage  is  that  it  is  impossible  to  secure  a  per- 
fectly electrically  balanced  winding  by  this  method.  This 
objection  does  not  hold  in  the  case  of  multipolar  gene- 
rators, hence  all  multipolar  drums  are  chord  wound. 

In  the  following  figures  the  numbered  radial  lines  will 
represent  armature  inductors,  the  lines  inside  of  them  will 


ARMATURES. 


53 


represent   their  connections  to  the  commutator  segments, 
and  the   lines  outside  of    them   will   represent  the   cross 
connections    between   inductors  at   the  pulley  end.     The 
brushes  are  placed  in- 
side   the     commutator 
for     convenience     and 
clearness. 

Fig.  35  represents  a 
six-circuit,  single  wind- 
ing with  80  inductors 
and  40  segments.  In 
practice  the  inductors, 
instead  of  all  lying  be- 
side each  other,  would 
probably  be  wound  one 
on  top  of  another  in 
one  slot. 

Fig.  36  shows  a  rather  simple  single  winding.     Although 

it  is  four  pole  it  is  but 
two  circuit,  in  which  it 
resembles  Fig.  37,  which 
is,  however,  a  triple  wind- 
ing. 

Fig.  38  gives  a  six- 
pole,  two  circuit,  double 
winding. 

In  winding  armatures, 
double  or  triple  cotton  in- 
sulated   copper    wire    is 
generally      used.        Care 
Fig'  36'  must  be  taken  to  well  in- 

sulate the  wires,  both  from  each  other  and  from  the  core. 


54 


DYNAMO  ELECTRIC   MACHINERY. 


Many  of   these    styles    of    winding   create  very  complex 

masses  of  wire  on  the 
ends  of  the  drum,  and 
great  care  must  be  ex- 
ercised both  in  regard  to 
insulating  and  fastening 
at  these  points,  so  that 
the  movement  of  the 
wires  under  the  influence 
of  the  "magnetic  drag" 
may  not  chafe  the  insu- 
lation and  short-circuit 
the  conductors.  Mica  is 
the  best  insulator,  and  is 
used  where  flat  sheets  are  needed  ;  but  its  great  cost,  and 
the  difficulty  of  manipulating  it,  result  in  the  extensive  use 
of  canvas,  oiled  paper,  rubber  tape,  vulcanized  fiber,  and 
many  patented  manufac- 
tured insulators.  Much 
reliance  is  placed  upon 
the  liberal  use  of  japan 
and  shellac,  especially  in 
conjunction  with  canvas. 
Where  very  large 
wires  are  used  on  the 
surface  of  an  armature, 
eddy  currents  are  set  up 
in  them  by  reason  of  one 
side  of  the  wire  being 
in  a  stronger  field  than  Fig*  38' 

the  other.     To  avoid  this  a  number  of  smaller  insulated 
wires  are  wound  in  parallel,  to  take  the  place  of  the  larger 


ARMATURES.  55 

one,  or  what  is  more  economical  of  space,  thin  copper  bars 
set  edgewise  take  the  place  of  the  round  wire. 

In  the  winding  of  multipolar  armatures  it  is  possible  to 
use  formed  coils,  which  are  wound  on  a  separate  collapsible 
forming  block,  and  are  afterward  applied  to  the  core.  This 
method  is  advantageous  in  that  better  insulation  can  be  as- 
sured, and  damaged  or  burned-out  coils  can  be  replaced 
without  disturbing  all  of  the  windings.  Fig.  39  shows  a 
General  Electric  Company's  formed  coil,  and  Fig.  40  some 
of  the  Crocker  Wheeler  coils. 


Fig.  39- 

All  armatures,  whether  wound  with  wire,  or  formed  coils, 
or  shaped  conductors,  must  be  banded  around  to  prevent 
dislodgement  of  the  conductors  under  influence  of  cen- 
trifugal action.  The  wire  used  for  this  purpose  is  gener- 
ally of  hard-drawn  brass  or  of  phospher  bronze,  and  on 
railway  motors  of  steel.  It  is  wound  over  insulating  strips 
forming  a  band  of  several  turns.  The  completed  turns 
are  often  sweated  together  with  solder. 

Many  manufacturers  punch  a  small  recess  in  each  side 
of  the  teeth  near  the  face.  A  strip  of  maple  wood  is  fitted 


DYNAMO   ELECTRIC   MACHINERY, 


ARMATURES. 


57 


Fi2. 


Fig.  42. 


into  the  recesses,  and  acts  like  a  cover  to  the  slot,  firmly 
holding  the  windings  in  place,  and  presenting  a  neat  ap- 
pearance. 

Figs.  41,  42,  43  show  respectively  a  core,  a  partially 
wound,  and  a  completed  General  Electric  Company's  arma- 
ture. Figs.  44  and  45  show  small  Westinghouse  types. 


DYNAMO  ELECTRIC  MACHINERY. 


Fig.  43. 


Fig.  44. 


Fig.  45- 


ARMATURES.  59 

39.  Commutators.  —  The  segments  or  bars  of  a  commu- 
tator are  always  of  drop-forged,  or  hard-drawn  copper. 
The  insulation  between  them  is  always  of  mica.  There 
are  various  grades  of  mica ;  and  for  insulating  purposes  the 
amber-colored  mica,  which  must  be  free  from  iron,  is  to  be 
preferred.  Besides  being  a  good  insulator,  amber  mica  has 
the  additional  advantage  that  it  wears  at  the  same  rate  as 
copper  ;  thus  after  long  use  it  leaves  neither  elevations  nor 
depressions  on  the  commutator  surface. 

In  fastening  the  bars  considerable  ingenuity  is  displayed  ; 
for  they  must  not  displace  themselves  with  reference  to 
the  windings,  neither  must  one  bar  lift  so  as  to  be  above 
the  level  of  its  neighbors.  If  the  latter  occurs,  then,  when 
the  bar  comes  under  a  brush,  it  will  lift  it ;  and  as  the 
high  spot  moves  out  from  under  the  brush  the  contact  is 
broken  until  the  spring  can  reseat  the  brush.  This  causes 
excessive  wear  and  destructive  sparking. 

After  a  commutator  has  been  for  a  time  in  use,  it  becomes 
grooved  and  pitted,  a  condition  which  causes  further  spark- 
ing and  wear,  and  the  commutator  must  be  turned  down 
again  to  a  true  surface.  The  design  of  a  commutator 
should  allow  of  good  operation  after  it  has  been  subjected 
to  this  treatment. 

Mechanical  friction  and  the  electrical  losses  that  accom- 
pany commutation  will  raise  the  temperature  of  the  com- 
mutator about  5°  C.  above  that  of  the  armature.  To 
secure  successful  operation  a  commutator  must  be  de- 
signed with  a  sufficient  number  of  bars,  so  that  the  differ- 
ence of  potential  between  two  adjacent  bars  shall  not 
exceed  10  volts.  This  would  mean  that  a  loo-volt  bi- 
polar machine  should  have  at  least  20  bars.  The  potential 
between  the  brushes  or  around  kalffae  commutator  is  100 


60  DYNAMO  ELECTRIC   MACHINERY. 

volts,  hence  half  the  commutator  must  have  10  bars. 
There  is  no  general  rule  for  the  length  of  a  commutator 
bar,  but  one  may  roughly  say  that  there  should  be  at  least 
one  inch  per  100  amperes. 

Commutators  should  be  designed  so  as  to  expose  a  suffi- 
cient area  to  radiate  the  heat  which  is  communicated  to 
them.  Except  in  the  case  of  some  special  commutators, 
which  are  supplied  with  cooling  devices,  at  a  peripheral 
speed  of  2,500  feet  per  minute,  the  radiation  of  one  watt 
per  square  inch  of  peripheral  radiating  surface  will  re- 
sult in  a  rise  of  temperature  of  20°  C.  The  permissible 
rise  of  55°  C.,  therefore,  allows  a  radiation  of  2.75  watts 
per  square  inch.  The  heat  to  be  radiated  is  due  to  the  fol- 
lowing causes : — 

a.  Friction  between  the  brushes  and   the   commutator 

/2    X    7T    X    746\ 

bars.     This  is  equal  to  I  -  —  1  times  the  product  of 

V      33'°°°      / 
the  following  quantities  :  The  radius  of  the  commutator  in 

feet,  the  speed  in  revolutions  per  minute,  the  coefficient 
of  friction  between  the  brushes  and  the  commutator  (0.3 
for  carbon  brushes  and  0.25  for  copper  brushes),  and  the 
sum  of  the  pressures  of  all  the  brushes  upon  the  commuta- 
tor. This  latter  should  amount  to  1.25  Ibs.  per  square  inch 
of  rubbing  surface.  Copper  brushes  permit  200  amperes 
per  square  inch  of  rubbing  surface,  and  carbon  brushes 
40  amperes. 

b.  The  contact  resistance  between  the  brushes  and  the 
commutator.     As  there  is  always  a  drop  of  about  one  volt 
at  each  point  of  contact,  and  as  there  is  a  drop  at  both  the 
positive  and  negative  terminals,  the  watts  represented  by 
these  contact  resistances  are  numerically  equal  to  twice  the 
current  of  the  machine. 


ARMATURES.  6l 

c.  The  energy  represented  in  the  sparking  at  the  brushes 
and  the  heat  due  to  waste  currents  in  the  short-circuited 
segments.  These  two  losses  cannot  be  accurately  calcu- 
lated, but  may  be  estimated  as  equal  to  about  6  per  cent  of 
the  total  commutator  loss. 


ItTica  collar-   under-  segmen 


Fig.  46. 

Fig.  46  gives  a  broken-away  view  of  a  General  Electric 
commutator,  showing  the  methods  of  attachment  and  insu- 
lation. 

40.  Collecting  Devices.  —  These  consist  of  the  brushes, 
the  brusJi  holders,  and  the  rockers. 

Brushes  for  high  potential  machines  are  of  carbon.  Car- 
bon against  copper  causes  less  wear  than  copper  against  cop- 
per, and  further,  the  greater  resistance  of  a  carbon  brush 
results  in  less  sparking  when  it  bridges  two  commutator  bars 
than  would  the  lower  resistance  of  a  copper  brush.  Com- 
bination brushes  of  carbon  and  copper  are  sometimes  used. 
Carbon  brushes  are  set  at  an  angle  generally,  though  some 
makers  set  them  radially  ;  and  in  motors  that  must  be  re- 


62  DYNAMO  ELECTRIC  MACHINERY. 

versed,  as  is  the  case  with  railroad  and  elevator  motors, 
they  are  invariably  set  radially.  A  surface  contact  of  one 
square  inch  per  40  amperes  is  usual  for  carbon  brushes. 

On  low-potential  machines  copper  brushes,  set  at  an  angle 
of  45°  with  the  tangent  to  the  commutator  surface  at  the 
point  of  contact,  are  invariably  used.    This  is  because  there 
is  less  natural  tendency  to  spark  on  low  voltages,  and  be- 
cause  the   resistance    of    carbon 
brushes    would    be    too    great    a 
fraction  of  the  whole  resistance  of 
the  circuit,  and  cause  a  wasteful 
drop  of  potential.     Copper  brushes 
must  have  their  ends  filed  to  give 
sufficient  surface  contact,  and  this 

is  generally  done  with  the  aid  of  &jig,  illustrated  in  Fig.  47. 
The  abrasion  of  carbon  brushes  is  accomplished  by  means 
of  glasspaper. 

Brush  holders  should  permit  of  a  low-resistance  contact 
between  the  brush  and  the  leads,  they  should  provide  ad- 
justment as  to  position  and  tension  of  the  brushes,  and 
they  should  be  arranged  so  that  none  of  the  springs  shall 
get  hot  and  lose  temper  while  in  performance  of  its  duties. 
The  tension  on  carbon  brushes  varies  from  i  to  10  Ibs.  per 
square  inch  of  contact  surface.  The  lower  limit  is  to  be 
found  in  large  central  station  generators,  and  the  higher 
limit  in  small  machines  and  in  motors  which  are  subjected 
to  frequent  and  sudden  strains,  as  railway  motors.  The 
coefficient  of  friction  between  brush  carbon  and  copper 
varies  from  0.28  to  0.32. 

Figs.  48  and  49  plainly  show  a  Crocker  Wheeler  rigging 
with  parallel-motion  brush  holders.  Fig.  50  shows  a  form 
of  General  Electric  holder. 


ARMATURES. 


Fig.  48. 


CLAMPING  SCREW 


BRUSH 
PRESSURE  SPRING 


ADJUSTING 
SCREW 


HARD  ROLLED  COPPER  LEAVES 


Fig.  49- 


64  DYNAMO   ELECTRIC    MACHINERY. 

Rockers  are  rings  or  attachments  carrying  the  brush 
holders,  and  they  are  mounted  concentric  with  the  com- 
mutator. They  are  made  to  give  all  the  brushes  of  the 
machine,  or  sometimes  all  the  positive  brushes  or  all  the 
negative  brushes  at  once,  a  motion  around  the  axis,  thus 
adjusting  all  brushes  by  one  movement.  Fig.  5 1  shows 
such  a  rocker. 

41.  Shafts,  Bearings,  and  Oilers.  —  Since  armature 
shafts  generally  have  high  speeds,  and  almost  always  are 
subject  to  sudden  large  variations  of  load,  the  shafts,  the 


Fig.  50. 

bearings,  and  the  oiling  facilities  must  be  well  designed. 
Wiener  gives  the  following  approximate  diameters  of  steel 
shafts  for  drum  armatures  :  — 

For       ioo  watts     ........       i  inch, 

For    1,000  watts 2    inches, 

For  10,000  watts 4!  inches, 

all  to  be  turned  down  at  the  bearings. 

It  is  necessary  that  the  bearing-boxes  be  exactly  in  line, 
and  a  form  of  self -alignment  bearing  is  frequently  used.  If 
undue  wear  in  the  bearings  occur,  the  armature  is  apt  to 


ARMATURES.  65 

sag  till  it  strikes  a  pole  piece,  which  will  damage  the  arma- 
ture. Many  machines  use  ordinary  oil  cups  to  secure 
lubrication,  while  others  make  use  of  some  device,  as  is 
shown  in  Fig.  52.  The  shaft  revolves  in  a  cylindrical  brass 
with  a  spherical  enlargement  at  its  middle  which  rests  upon 


P~ .,/    '    r"j"  <r-^*5 
I    *  CT  U  '\.m*£r  A 


Fig.  51. 

a  corresponding  spherical  bed  of  Babbit  metal.  This  se- 
cures self-alignment.  Two  slots  are  cut  radially  in  the 
brass,  and  allow  two  rings  to  rest  upon  the  shaft.  These 
rings  are  also  of  brass,  and  have  an  inside  diameter  slightly 
larger  than  the  outside  diameter  of  the  brass  cylinder. 


66 


DYNAMO   ELECTRIC   MACHINERY. 


The  pillow  block  is  hollowed  away  under  these  rings,  the 
hollows  serving  as  receptacles  for  the  storage  of  oil.     As  the 


Fig.  52. 


shaft  revolves,  the  rings  also  revolve  at  such  a  rate  as  to 
carry  a  steady  stream  of  oil  up  into  the  slots,  thereby 
lubricating  the  bearing. 


FIELD   MAGNETS. 


CHAPTER  IV. 

FIELD  MAGNETS. 

42.  Parts  of  Field  Magnets.  —  The  parts  of  a  dynamo, 
exclusive  of  the  armature,  which  make  up  the  magnetic 
circuit,  belong  to  the  field  magnets.  Fig.  53  shows  a  con- 
ventional bipolar  horse-shoe  type  with  the  parts  plainly 
marked.  The  field  cores  are  the  iron  centers  in  the  mag- 
netizing coils.  The  yoke  connects  the  cores  together  at 
one  end  while  the  other  ends  terminate  in  the  pole  pieces, 


Fig.  54- 

one  being  a  north  magnetic  pole,  the  other  a  south.  The 
side  of  the  pole  piece  embracing  the  armature  is  styled  the 
pole  face,  and  the  latter's  projecting  edges  are  fittingly 
called  the  horns.  Some  dynamos  have  the  magnetizing 
coils  on  the  yoke,  thus  making  the  latter  serve  also  as 
core.  In  different  types  different  numbers  of  pieces  pre- 
vail, thus  all  the  parts  (save  the  coils)  might  be  cast  in 
one  piece  or  each  might  be  made  separately. 


68  DYNAMO   ELECTRIC   MACHINERY. 

In  multipolar  machines  the  designation  of  the  parts  is 
somewhat  different  than  in  the  case  of  bipolar  machines. 
The  particular  designation  often  depends  upon  the  manu- 
facturer. Fig.  54  gives  the  designation  used  by  the 
Crocker  Wheeler  Company. 

43.  Magnetic  Material. — The  materials  used  for  field 
magnetic  circuits  are  three,  —  cast  iron,  wrought  iron,  and 
cast  steel.  The  selection  of  material  for  a  given  machine 
is  governed  by  considerations  of  (a)  weight,  (b]  first  cost, 
(c)  economy  and  satisfactory  regulation  when  in  operation. 

Cast  iron  has  the  great  advantage  of  cheapness ;  but  it  is 
poor  magnetically,  hence  more  weight  and  bulk  must  be 
employed  to  perform  the  same  service  as  the  magnetically 
superior  wrought  iron.  It  costs  more  in  copper  to  magne- 
tize a  cast-iron  core,  because  more  turns  will  be  required, 
and  each  turn  will  be  longer  than  if  the  core  were  of  better 
material. 

Wrought  iron  is  the  best  magnetic  material  available. 
It  is  used  either  in  forgings,  or  in  the  form  of  plates 
punched  from  the  sheet.  In  either  form  it  is  expensive ;  but 
since  less  weight  in  a  given  machine  is  necessitated  when 
this  metal  is  used,  it  is  often  chosen  where  portability 
is  required,  as  in  the  case  of  the  marine  dynamos,  electric 
railroad  motors,  and  particularly  motors  for  automobiles. 

Cast  steel  is  intermediate  between  cast  iron  and  wrought 
iron,  both  in  cost  and  in  magnetic  properties,  and  is  much 
employed  in  good  practice.  The  use  of  different  metals  in 
different  parts  of  the  frame  is  very  general.  For  instance, 
a  cast-iron  yoke  is  used  with  cast-steel  cores  and  pole 
pieces,  or  a  cast-iron  or  steel  yoke  is  used  with  wrought- 
iron  cores  and  pole  pieces. 


FIELD    MAGNETS.  69 

44.  Shape  of  Field  Magnets.  —  There   is  a  great  vari- 
ety of  shapes  of  field  magnets.      Formerly  each  manufac- 
turer had  a  type  peculiarly  his  own,  and  this  led  to  many 
forms,  some  of  little  merit.     These  freak  types  are  now 
disappearing,  and  a  few  general  types  are  adopted  more  or 
less  by  all  makers.     In  all  forms,  however,  the  polar  span, 
or  part  of  the  armature  circle  that  is  covered  by  pole  faces, 
is  from  65  per  cent  to  75  per  cent,  or  from  234°  to  270°. 
In  general  a  small  number  of  poles  in  the  field  magnets  re- 
quires less  copper  in  the  exciting  coil  than  does  a  larger 
number,  and  also   the  fields  can  be  excited  more  economi- 
cally.    But  in  large  bipolar  machines  successful  operation 
under  varying  loads  requires  a  large  air  gap  between  the 
pole  face  and  the  armature.     This  increases  the  magnetic 
reluctance  and  the  energy  necessary  for  excitation.     Multi- 
polar  machines  do  not  require  so  large  an  air  gap.    Further- 
more, increasing  the  number  of  poles  gives  the  mechanical 
advantage  of  allowing  a  lower  armature  speed  without  low- 
ering the  potential  of  the  output.      Multipolar  machines 
will   run    cooler   than  bipolars    of   the   same  economy  of 
operation. 

Speaking  generally,  though  it  is  by  no  means  a  rule, 
bipolar  fields  are  used  up  to  about  10  K.W.,  four-pole  fields 
from  10  K.W.  to  100  K.W.,  six-pole  fields  from  100  K.W.  to 
300  K.W.,  and  beyond  that  point  eight  or  more  poles  are 
generally  used. 

45.  Methods  of  Excitation   of  Fields.  —  Dynamos   are 
classified  according   to  the   five  methods   of  exciting  the 
fields  of   the    machine.      They  are :  —  the  Magneto,   the 
Separately  Excited,  the   Shunt  Wound,  the  Series  Wound, 
and  the  Compound  Wound. 


DYNAMO   ELECTRIC   MACHINERY. 


The  magneto  generator,  Fig.  55,  is  one  in  which  the 
field  is  a  permanent  steel  magnet,  generally  of  horse-shoe 
type. 

The    separately  excited    dynamo,    Fig.    56,   has,   as    its 


MAGNETO  DYNAMO 
Fig.    55- 


SEPARATELY  EXCITED  DYNAMO 

Fig.  56. 


name  implies,  its  field  coils  traversed  by  a  current  other 
than  that  produced  by  the  machine.  Alternating  current 
machines  are  nearly  always  of  this  type. 

The  shunt-wound  machine,  Fig.  57,  has  a  large  number 
of  turns  of  fine  wire  wound  on  its  core,  and  the  ends  are 


SHUNT  WOUND 
DYNAMO 

Fig.  57- 


SERIES  WOUND 
DYNAMO 

Fig.  58. 


connected  to  the  terminals  of  the  machine,  thus  being  in 
shunt  with  the  outside  circuit.  The  ampere  turns  requisite 
for  excitation  are  obtained  by  passing  a  small  number  of 
amperes  through  a  large  number  of  turns. 

The  series-wound  generator,    Fig.    58,   has  all  the  cur- 


FIELD    MAGNETS. 


rent  that  is  produced  by  the  armature  passed  through 
large  conductors  wound  with  fewer  turns  around  the  cores. 
The  exciting  coils  are  then  in  series  with  the  external  cir- 
cuit. The  ampere  turns  required  for  excitation  are  ob- 
tained by  passing  a  large  current  through  a  small  number 
of  turns. 

The  compound  machine,  Fig.  59,  is  one  in  which  there 
are  both  shunt  and  series  coils  on  the  field  magnets.  This 
method  of  winding  is  used  for  purposes  of  regulation  under 
varying  loads,  as  will  be  explained  later.  Compound  wind- 
ings are  of  two  classes,  the  long  shunt  and  the  short  sJiunt. 
In  the  former,  the  current  used  in  the  shunt  windings  is 


COMPOUND    WOUND 
DYNAMO  LONG  SHUNT 

Fig.  59- 


COMPOUND    WOUND 
DYNAMO  SHORT  SHUNT 

Fig.  60. 


also  passed  through  the  field  windings  along  with  the  main 
current.  In  the  latter,  the  current  from  the  shunt  coils 
passes  directly  back  to  the  armature,  avoiding  the  series 
turns.  Figs.  59  and  60  clearly  show  the  two  methods. 
The  short  shunt  is  generally  preferred. 

46.  Field  Coils.  —  The  coils  of  a  dynamo  must,  without 
undue  elevation  of  temperature,  supply  sufficient  ampere 
turns  to  give  the  required  excitation.  This  temperature 
rise  will  not  be  excessive  when  about  o.  3  5  watts  are  radiated 
per  square  inch  of  outer  surface  of  the  coil.  If  no  account 
be  taken  of  the  ends  of  the  pole  and  coil,  0.6  watt  may  be 


72  DYNAMO   ELECTRIC   MACHINERY. 

allowed  per  square  inch.  The  field  coils  have  no  ventila- 
tion due  to  their  own  motion  as  have  armatures,  hence 
about  1000  circular  mils  per  ampere  must  be  allowed  in 
the  wire  which  composes  such  coils.  The  cost  of  copper 
is  needlessly  increased,  if  more  than  the  necessary  cross- 
section  be  allowed. 


Fig.  61. 

Field  coils  are  usually  wound  on  brass  or  iron  spools, 
shaped  to  slip  over  the  cores.  Sometimes,  especially  in  the 
case  of  small  machines,  the  coils  are  wound  on  frames, 
which  can  be  collapsed  and  removed.  The  coils  of  series 
machines  and  the  series  coils  of  compound  machines  are 


FIELD   MAGNETS. 


73 


often  wound  with  copper  ribbon  instead  of  wire,  or  are  even 
made  up  of  forged  copper  conductors,  having  a  rectangular 
cross-section.  This  is  because  the  heavy  currents  require 
such  large  cross-section  of  conductor  that  if  made  of  wire 
much  space  would  be  lost  between  the  wires.  The  rear 
coil  in  Fig.  61  is  a  series  coil  of  shaped  conductors.  This 
figure  shows  both  the  shunt  and  the  series  coil,  as  wound 
by  the  Westinghouse  Company,  for  a  compound  multipolar 
railway  generator.  The  binding  which  is  seen  on  the  shunt 
coils  in  both  illustrations  should  not  be  mistaken  for  the 
wires  of  these  coils.  Field  coils  are  wound  with  double 
cotton-covered  copper  wire.  Further  insulation  between 
coil  and  core,  and  between  series  and  shunt  coils,  is  effected 
by  the  use  of  fiber,  fuller  board,  and  mica. 

47.  Magnetic  Leakage.  —  Since  air  is  not  an  insulator 
of  magnetism,  but  is  simply  much  less  permeable  than 
iron,  it  is  evident  that  some  of 
the  lines  of  force  generated  by 
the  field  coils  will  not  follow 
around  the  desired  path  through 
pole  pieces  and  armature,  but  will 
take  a  path  through  the  air  and 
be  of  no  utility  in  creating  E.M.F. 
in  the  revolving  armature.  Fig. 
62  roughly  represents  some  of 
the  paths  such  lines  may  take. 

If  $t  be  the  total  flux  caused  by  the  field  coils  and  <£rt  be 
the  flux  that  passes  through  the  armature,  then  the  coeffi- 
cient of  magnetic  leakage, 

,    *, 
=*.' 

and  is  always  greater  than  unity. 


Fig.  62. 


74 


DYNAMO  ELECTRIC   MACHINERY. 


In  practice  X.  varies  from  1.25  to  1.4  in  single  horse- 
shoe fields,  and  in  the  Edison  type  of  inverted  horse-shoe 
and  in  double  horse-shoe  fields  it  varies  from  1.5  to  1.75. 
In  multipolar  machines  X  varies  from  i.i  to  1.5. 

To  find  the  coefficient  of  magnetic  leakage  of  small  or 
moderate  sized  machines  proceed  as  follows  :  — 

Arrange  the  field-coils  for  separate  excitation  by  a  cur- 
rent that  can  be  conveniently  commutated.  Suppose  the 
machine  to  have  a  field  of  the  double  horse-shoe  type,  as  in 
Fig.  63.  Take  a  few  turns  of  fine  insulated  wire  about 
the  middle  of  one  coil,  as  c,  d,  and  connect  the  ends  to  a 


Fig.  63. 

ballistic  galvanometer  of  low  sensioility.  A  low-reading 
Weston  voltmeter  will  answer.  Suddenly  commutate 
the  current  in  the  field  coils.  The  change  in  direction  of  the 
flux  in  the  core,  from  -f  <j>  to  —  <f>,  will  induce  E.M.F.  in  the 
test  coil,  which  will  give  a  throw  to  the  voltmeter  needle. 
The  deflection  is  directly  proportional  to  the  flux  in  the 
core.  Repeat  with  the  other  coil,  and  the  sum  of  the  de- 
flections obtained  from  cd  and  ef  is  directly  proportional 
to  the  total  flux  produced  <£r  Now  make  a  test  coil  of  the 
same  number  of  turns  and  of  the  same  resistance  about 
the  armature,  in  such  a  position  ab  that  it  includes  the  area 


FIELD   MAGNETS.  7$ 

of  the  armature  that  is  cut  by  the  greatest  number  of  lines 
of  force.  Upon  commutating  tr  e  current  a  throw  of  the 
needle  will  result,  which  is  propcitional  to  the  flux  in  the 
armature  <£a.  Hence  the  coefficient  of  magnetic  leakage, 

<#>t  _  defl.  at  cd  +  defl.  at  ef 
<f>a       deflection  at  armature 

The  exciting  current  must  remain  constant  during  the 
investigation. 

The  location  of  the  different  leakage  paths  may  be  found 
by  using  test  coils  on  different  parts  of  the  frame.  The 
difference  between  the  throws  observed  at  any  two  places 
is  a  measure  of  the  leakage  between  those  two  places. 

Clearly  the  number  of  lines  choosing  paths  through  the 
air  will  decrease  as  the  permeability  of  the  iron  circuit 
increases.  An  increase  in  the  reluctance  of  the  main 
magnetic  circuit  will  increase  the  leakage  loss. 

Armature  cores  vary  in  permeability  under  varying  con- 
ditions of  load.  As  the  load  increases,  this  change  pro- 
duces an  increase  in  the  reluctance  of  the  main  magnetic 
circuit.  This  results  in  an  increase  of  the  loss  by  leakage. 
The  coefficient  of  magnetic  leakage  is,  therefore,  different 
with  different  loads. 

48.  Pole  Pieces  and  Shoes.  —  In  general  practice  the 
field  cores  and  the  frame  of  a  generator  are  worked  at  a 
flux  density  of  at  least  15,000  lines  per  sq.  cm. 

This  is  too  high  a  value  to  use  in  the  air  gap.  Therefore 
pole  shoes  are  put  on  the  ends  of  the  pole  pieces  to  dis- 
tribute this  flux  over  a  wider  area  where  it  has  to  pass 
through  the  air,  and  to  thus  decrease  the  total  reluctance 
of  the  magnetic  circuit. 


76  DYNAMO   ELECTRIC   MACHINERY. 

49.  Effect  of  Joints  in  the  Magnetic  Circuit.  —  Since  no 
two  pieces  of  metal  can  be  put  together  with  a  perfect 
joint,  there  is  always  an  increase  of  reluctance  in  a  mag- 
netic circuit  when  a  joint  is  introduced  therein.  Professor 
Ewing  found  by  experiment  that  at  low  magnetizations 
(3C  =  7.5)  the  increase  of  reluctance  of  a  certain  bar  of 
iron  due  to  a  joint  was  above  20  per  cent,  and  that  for 
high  magnetizations  (3C  =  70)  the  loss  due  to  one  joint  was 
less  than  5  per  cent.  The  difference  is  probably  due  to 
the  fact  that  the  pieces  under  strong  magnetizations  attract 
themselves  so  powerfully  as  to  make  a  more  perfect  joint. 
Ewing  also  found  that  a  single  cut  in  a  bar  acted  upon  the 
reluctance  of  the  bar  as  though  the  length  of  the  bar  had 
been  increased  by  amounts  given  in  the  following  table :  — 

For3C  = 7.5       15          30  50  70 

Equivalent  length  of   i  cut 

in  cms.  of  iron     ...       4      2.53       i.io       0.43        0.22 


OPERATION   OF  ARMATURES.  77 


CHAPTER   V. 

OPERATION    OF   ARMATURES 

50.  Process  of  Commutation.  —  The  simple  process  of 
commutation  as  described  in  §  30  is  attended  with  some 
difficulties  in  practice.  Consider  one  coil  of  a  plain  ring 
armature  with  the  commutator  bars  attached  thereto  as  in 
Fig.  64.  In  position  A,  when  the  brush  is  on  only  one  of 
the  bars  in  question,  the  action  of  the  other  coils  of  the 
armature  will  be  to  force  current  in  this  one  coil  in  the 
direction  indicated  by  the  arrow.  B  is  considered  to  be 


the  positive  brush.  In  position  D,  when  the  brush  has 
passed  over  to  the  other  bar  entirely,  the  direction  of  the 
current  in  this  coil  is  in  the  other  direction.  Now  this 
change  of  direction  must  occur  when  the  coil  is  in  a  weak 
field,  for  it  is  observed  that  the  coil  is  short  circuited  while 
in  position  C,  the  circuit  being  completed  through  the  coil, 
the  bars  and  the  brush  spanning  the  mica  insulation  at  o. 
If  now  at  this  moment  the  coil  should  be  in  a  strong  field, 
and  should  be  cutting  many  lines  of  force,  too  large  an 


DYNAMO   ELECTRIC   MACHINERY. 


E.M.F.  would  be  produced,  and  as  the  resistance  of  the 
circuit  indicated  is  very  low,  an  excessively  strong  current 
might  flow.  When  the  brush  slips  past  o  the  circuit  is 
broken,  and  a  more  or  less  serious  sparking  occurs  accord- 
ing to  the  strength  of  the  current  flowing  at  the  instant  of 
break.  Commutation  must  then  be  effected  when  the  coil 
is  in  such  a  position  as  not  to  cut  many  lines  of  force.  It 
follows  that  every  commutating  machine  must  have  at 
least  two  places  where  the  effective  field  has  a  zero  value. 
Fig.  65  gives  a  rectified  curve  of  the  magnetic  distribution 


Fig.  65. 

under  the  pole  pieces  and  around  the  armature  of  a  well- 
designed  bipolar  machine,  the  ordinates  of  the  curve  giving 
the  flux  density  in  the  air  gap. 

The  neutral  plane  is  a  plane  passed  through  the  axis  of 
the  armature  and  a  point  in  the  field  immediately  surround- 
ing the  armature,  where  the  inductively  effective  com- 
ponent has  a  zero  value.  The  coil  in  position  C,  Fig.  64 
is  supposed  to  be  in  the  neutral  plane. 

The  commutating  plane  is  a  plane  passed  through  the  axis 
of  the  armature  and  through  the  points  of  contact  of  the 
brushes.  The  segments  are  supposed  to  be  connected  with 
parts  of  the  armature  windings  lying  on  the  same  radius. 


OPERATION    OF   ARMATURES.  79 

51.    Influence    of    Self-induction    of    the    Commutated 

Coil.  —  When  the  coil  in  Fig.  64  is  in  position  A  the  cur- 
rent flowing  in  it  produces  magnetic  flux  in  the  ring  inde- 
pendent of  any  inductive  action  of  the  field  magnets  of  the 
dynamo,  and  links  the  flux  with  itself.  When  the  coil  is 
in  position  D,  there  is  also  a  magnetic  flux  and  linkage,  but 
its  direction  has  been  changed.  Therefore,  in  passing 
through  the  position  C,  the  current  in  the  coil  and  the 
accompanying  flux  linked  with  the  coil  have  decreased  to 
zero,  and  have  afterwards  risen  in  value  in  the  opposite 
direction. 

This  change  of  flux  produces  an  E.M.F.  in  the  coil  inde- 
pendent of  any  action  of  the  field  magnets  (see  §  15).  This 
E.M.F.  is  called  an  electromotive  force  of  self-induction  and 
tends  to  continue  the  flow  of  a  current  which  has  been 
started,  and  tends  to  prevent  any  increase  or  decrease  in 
the  strength  of  the  current  and  to  prevent  the  stopping  or 
starting  of  the  current.  The  value  of  this  self-induced 
pressure  with  a  given  flow  of  current  varies  as  the  square 
of  the  number  of  turns  in  the  coil,  as  the  cross-section  of 
the  coil,  and  as  the  permeance  of  the  magnetic  circuit. 
Because  of  self-induction  it  is  evident  that,  if  commutation 
take  place  in  the  neutral  plane,  there  is  a  liability  that  it 
will  be  accompanied  by  excessive  currents  in  the  short-cir- 
cuited coils  and  consequently  by  sparking.  This  trouble 
is  to  be  avoided  by  revolving  the  plane  of  commutation 
about  the  shaft  of  the  machine  until  a  sufficiently  strong 
field  acts  upon  the  short-circuited  coil  to  induce  an  opposing 
E.M.F.  of  the  same  value  as  the  E.M.F.  of  self-induction. 
Both  the  self-induced  E.M.F.  and  the  E.M.F.  due  to  the 
rotation  of  the  armature  vary  in  magnitude  during  the 
time  that  a  brush  is  upon  two  adjacent  segments.  Their 


8o 


DYNAMO   ELECTRIC   MACHINERY. 


\ 


Fig.  66. 


manners  of  variation  need  not  be  alike,  and  hence  it  may 
be   impossible    in    some    cases   to   effectively  oppose  one 

against  the  other.  The 
obvious  remedy  is  to  be 
sought  in  more  com- 
mutator segments  or  a 
change  of  shape  of  pole 
shoe. 

52.  Cross-Magnetiz- 
ing Effect  of  Armature 
Currents.  —  Indepen- 
dent of  field  magnets 
the  current  flowing  in 
the  armature  conductor 
will  magnetize  the  ar- 
mature core.  The  poles  thus  produced  will  be  in  the 
plane  of  commutation.  Fig.  66  shows  the  magnetizing 
effect  of  the  armature  turns 
on  a  ring  armature.  Fig. 
67  shows  a  cross-section  of 
a  drum  armature  and  its 
windings  with  the  resulting 
magnetization. 

Thus,  when  there  is  a 
load  on  a  dynamo  and  the 
armature  conductors  are 
carrying  a  heavy  current, 
there  are  two  coexistent 
magnetic  fields.  This  condition  results  in  a  skewing  of 
the  lines  of  force,  as  is  shown  in  Fig.  68.  As  the  lines 
are  skewed  the  neutral  plane  is  shifted.  To  produce  spark- 


Fig.  67. 


OPERATION   OF  ARMATURES. 


81 


less  commutation  the  commutating  plane  must  also  be 
shifted.  This  causes  a  further  skewing  of  the  lines.  The 
limit  of  this  double  interdependent  shifting  is  reached 
when  the  magnetic  lines  have 
become  so  crowded  in  the 
trailing-pole  tips  that  they  are 
almost  insensible  to  a  further 
shifting  of  the  plane  of  com- 
mutation. 

This  skewing  is  a  source  of  loss  in  the  operation  of  a 
generator  because  it  increases  the  magnetic  reluctance  in 
two  ways,  —  (a)  by  saturating  the  iron  at  the  horns,  and 
thus  reducing  the  permeability,  and  (b)  by  lengthening  the 
paths,  both  in  air  and  in  iron,  that  the  lines  must  follow. 

Fig.  69  shows  a 
curve  similar  to  Fig. 
65  taken  when  the  gen- 
erator was  under  load 
and  the  armature  was 
traversed  by  a  heavy 
—  current,  the  flux  being 
distorted  because  of  it. 
It  is  evident  that  the 
angular  displacement  of 
the  neutral  plane  depends  in  magnitude  upon  the  relative 
number  of  armature  ampere  turns  as  compared  with  the  ef- 
fective field  ampere  turns.  The  use  of  a  strong  field  and  a 
large  air-gap  length  requires  a  large  number  of  field  ampere 
turns.  Both  are  much  used  in  practice  with  great  success. 

53.  Demagnetizing  Effect  of  Armature  Currents.  —  It 
has  been  shown  that  it  is  necessary  to  have  the  commu- 


Fig.  69. 


82 


DYNAMO   ELECTRIC  MACHINERY. 


tating  plane  in  advance  of  the  neutral  plane.  The  angle 
between  them  is  called  the  angle  of  lag  or  lead.  If  an 
axial  plane  be  passed  through  the  armature,  making  with 
the  neutral  plane  an  angle  equal  to  the  angle  of  lag  or  lead, 
but  on  the  opposite  side  of  the  neutral  plane  from  the  corn- 
mutating  plane,  then  the  angular  space  between  this  plane 
and  the  commutating  plane  is  called  the  double  angle  of 
lag  or  lead.  The  armature  conductors,  which  create  a 
magnetism  that  tends  to  skew  the  lines  of  the  field  magnets 
as  shown  in  the  last  article,  are  called  the  cross  turns. 


They  lie  outside  the  double  angle  of  lead.  Those  armature 
conductors  which  lie  within  the  double  angle  of  lead  are 
called  the  back  turns,  because,  when  carrying  a  current, 
their  magnetic  tendency  is  to  send  lines  in  a  direction 
exactly  opposite  to  the  lines  of  the  field  magnets.  They 
neutralize  in  a  certain  measure  the  action  of  the  field  turns. 
This  action  is  clearer  shown  in  Fig.  70,  which  is  a  cross- 
section  of  a  bipolar  drum  armature.  At  a  there  is  a  north 
pole  due  to  the  back  turns  which  lie  in  the  double  angle, 
and  at  b  there  is  the  corresponding  south  pole.  The  effect 


OPERATION   OF  ARMATURES.  83 

of  these  poles  is  to  neutralize  some  of  the  useful  magnetic 
lines  flowing  from  N  to  S.  At  c  there  is  a  south  pole 
due  to  the  remaining  or  cross  armature  turns  and  at  d  is 
the  corresponding  north  pole.  These  poles  skew  the  lines 
flowing  from  N  to  S.  Compensation  for  back  turns  is 
easily  calculated,  since  the  number  of  back  turns  times 
the  current  in  them  at  any  load  multiplied  by  the  coeffi- 
cient of  magnetic  leakage  at  that  load  (§  47)  gives  the 
number  of  additional  field  ampere  turns  necessary  at  that 
load  for  compensation. 

54.  Sparking.  —  As  shown  in  §  51,  sparking  can  be 
avoided  by  giving  the  brushes  a  lead  sufficient  to  bring  the 
coils  they  short  circuit  into  fields  sufficiently  strong  to  coun- 
teract the  effects  of  self-induction.  Sparking  in  the  opera- 
tion of  machines  is  generally  due  to  the  misplacement  of  the 
brushes,  though  sometimes  it  is  due  to  irregularities  of  the 
commutator  surface.  A  high  bar  passing  from  under  a 
brush  will  leave  the  latter  suspended  in  air  a  moment,  which 
will  break  the  whole  current  through 
the  brush  and  cause  a  bad  spark  or 
arc. 

A  machine  may  also  suffer  melting 
of  the  commutator  bars  without  any 
visible  sparking.  Suppose  a  coil  of 
low  resistance  to  be  short  circuited 
by  a  copper  brush  as  in  Fig.  71. 
When  the  brush  is  chiefly  on  one 
bar,  and  over-laps  the  other  very 
slightly,  then  a  very  considerable  part  of  the  resistance  in 
the  circuit  is  the  transition  resistance  at  the  small  contact. 
Under  an  E.M.F.  of  self-induction  a  current  of  sufficient 


84  DYNAMO   ELECTRIC   MACHINERY. 

magnitude  may  flow  to  produce  enough  heat  in  the  trans- 
ition resistance  to  melt  the  surface  of  the  commutator  bar. 
The  E.M.F.  may  then  disappear  before  the  brush  leaves 
the  bar,  and  there  will  be  no  spark  visible. 

Sparking  may  be  due  to  excessive  electromotive  force 
between  the  commutator  segments  undergoing  commuta- 
tion due  to  the  self-induction  of  the  coil  and  to  mutual 
induction  between  it  and  other  coils  undergoing  commuta- 
tion at  the  same  time.  To  be  able  to  determine  the  value 
of  this  induced  E.M.F.  one  must  know  both  the  self  and 
mutual  inductances,  and  the  time  rate  of  suppression  of  the 
current  in  the  coil.  Parshall  and  Hobart  state  that  in 
practice  one  may  assume  that  a  coil  of  a  single  turn  when 
traversed  by  one  ampere  produces  and  links  with  itself 
20  c.g.s.  lines  per  inch  net  length  of  armature  lamination. 
From  this  datum  one  can  calculate  the  values  of  the  self- 
inductance  and  mutual  inductance. 

A  coil  which  is  undergoing  commutation  must  have  its 
current  changed  from  a  maximum  value  in  one  direction  to 
zero  and  from  zero  to  a  maximum  value  in  the  other  direc- 
tion during  the  time  that  the  two  segments  at  its  ends  are 
connected  through  the  brush.  This  time  is  evidently 
dependent  upon  the  peripheral  speed  of  the  commutator 
and  upon  the  width  of  the  brush.  It  is  equal  to  the  time 
that  it  takes  a  point  of  the  insulation  between  the  seg- 
ments to  pass  over  the  breadth  of  the  brush ;  that  is,  the 
time  in  seconds  is  equal  to  the  breadth  of  the  brush  in 
inches  divided  by  the  peripheral  velocity  of  the  commutator 
in  inches  per  second.  The  reciprocal  of  this  time  gives  the 
number  of  commutations  per  second,  or  what  is  termed 
the  frequency  of  commutation.  The  frequencies  found  in 
practice  lie  between  200  and  500  per  second.  While  all 


OPERATION    OF   ARMATURES.  85 

the  current  which  traverses  the  coil  is  suppressed  in  one- 
half  the  time  taken  for  commutation,  the  manner  of  its 
variation  is  unknown.  Parshall  and  Hobart  assume  that 
the  current  strength  falls  sinusoidally.  An  assumption  of 
a  uniform  decrease  with  the  time  yields  results  quite  in 
accord  with  practice.  The  value  of  the  induced  voltage 
then  will  be  equal  to  the  product  of  the  value  of  the  corn- 
mutated  current  and  the  sum  of  the  mutual  and  self-induc- 
tance divided  by  one-half  the  time  occupied  in  completing 
commutation.  This  value  should  not  exceed  6  volts. 

55.  Prevention  of  Sparking The  limit  of  the  capacity 

of  a  machine  may  be  excessive  sparking  instead  of  exces- 
sive heating,  and  therefore  the  suppression  of  sparking  by 
proper  design  of  the  machine  is  of  utmost  importance. 

Sparking  may  be  prevented  :  — 

a.  By  shifting  the  brushes  till  the  short-circuited  coil 
is  just  under  the  fringe  of  the  pole  piece.     This  counter- 
acts the  effects  of  self-induction  as  explained  in  §  51.     The 
reversal  of  the  direction  of  flux  in  any  but  the  short-cir- 
cuited coils  is  to  be  avoided,  since  a  loss  of  useful  E.M.F. 
would  then  occur. 

b.  By  having  a  stiff  field,  that  is,  a  field  so  strong  as  to 
suffer  very  little  skewing  because  of  the  armature  cross 
turns.     There  is  then  no  lag.     In  practice,  air-gap  magnetic 
densities  vary  from  2500  to  7500  lines  per  square  centimeter. 
The  higher  densities  are  to  be  found  in  the  larger  machines. 
There  is  a  general  tendency  to  increase  the  density. 

c.  By  nearly  saturating  the  teeth  of  the  armature  core. 
When  the  core  teeth  are  nearly  saturated,  an  increase  of 
load  increases  the  reluctance  very  markedly,  and  the  demag- 
netizing effect  of  the  back  turns  is  restrained  on  increase 


86  DYNAMO   ELECTRIC   MACHINERY. 

of  load,  because  of  the  greater  reluctance  of  the  circuit. 
This  minimizes  the  shift  of  the  commutating  plane  from  no 
load  to  full  load,  and  is  a  device  invariably  employed  on 
railway  generators  and  other  machines  that  have  to  stand 
severe  changes  of  load  without  change  of  position  of 
brushes. 

d.  By  using  brushes  of  carbon,  brass  gauze,  etc.      In 
machines  of   over    100  volts,  carbon  brushes   are   always 
used.     Besides  their  good  wearing  qualities,  their  resistance 
prevents  the  flow  of  a  large  current  in  the  short-circuited 
coil  in    commutation,    and   thus   a   misplacement    of    the 
brushes  will  not  result  in  so  violent  a  spark.     In  very  low- 
potential  machines,  as  has  already  been  said,  carbon  brushes 
are    impracticable,    because   their  resistance   causes  a  too 
great  fall  of  potential.     So  in  these  machines  copper  strip 
brushes  are  employed  when  possible.     When  too   much 
sparking  occurs  with  plain  copper  brushes,  a  brush  of  some- 
what greater  resistance  is  employed,  such  as  copper  gauze, 
brass,  brass  gauze,  etc.,  according  to  the  requirements  of 
the  case. 

e.  By  slotting  the  pole  pieces  longitudinally.     This  in- 
creases the  reluctance  offered  to  the  lines  due  to  armature 
reactions,  and  so  tends  to  prevent  sparking. 

•f.  By  properly  shaping  the  pole  pieces.  The  distribu- 
tion of  flux  should  be  such  that  a  coil  enters  a  weak  field 
first,  and  so  gradually  comes  to  the  strongest  part.  If  the 
lines  of  force  are  allowed  to  crowd  into  the  trailing-pole 
tips,  this  gradual  transition  is  impossible.  If  the  horns  are 
farther  from  the  armature  surface  than  the  body  of  the 
pole  face,  then  the  air  gap  and  consequently  the  reluctance 
at  the  horns  is  increased,  and  the  lines  are  compelled  to 
distribute  themselves  more  symmetrically.  A  place  suit- 


OPERATION   OF  ARMATURES.  87 

able  for  commutation  is  then  more  readily  found.  One 
may  also  resort  to  the  shaping  of  the  pole  pieces  by  champ- 
fering  the  corners,  or  by  making  the  pole  faces  with  a  cir- 
cle of  greater  radius  than  the  armature. 

The  Sprague  Electric  Company,  in  its  split-pole  type  of 
the  Lundell  generator,  avoids  the  distortion  of  the  field 
under  full  load,  due  to  cross  magnetizing  turns,  by  making 
use  of  a  specially  designed  pole  piece.  Fig.  72  repre- 
sents a  cross-section  of  this  generator,  and  shows  the  con- 
struction of  the  pole  piece.  The  magnetic  flux  which 
enters  the  pole  piece,  divides  between  the  two  paths  a  and 
b.  Owing,  however,  to  the  greater  span  covered  by  the 
shoe  belonging  to  the  part  marked  b,  the  magnetic  reluc- 
tance of  that  part  is  much  smaller  than  that  of  the  part 
marked  a.  As  a  result,  the  flux  does  not  divide  itself 
equally  between  the  two  paths.  The  part  of  the  pole  piece 
marked  by  under  increasing  excitation  becomes  saturated 
before  the  part  marked  a.  At  normal  excitation,  the  flux 
density  at  b  is  above  16,000  lines  per  square  centimeter, 
while  the  flux  density  in  a  is  but  about  10,000  lines  per 
square  centimeter.  In  other  words,  b  is  pretty  well  satu- 
rated, while  a  has  not  been  brought  to  a  magnetization  as 
high  as  the  knee  of  the  magnetization  curve.  This  satura- 
tion of  half  of  the  pole  piece  is  effective  in  preventing  a 
skewing  of  the  field  by  the  cross  turns.  This  is  shown  in 
Figs.  73  and  74,  where  Fig.  73  represents  the  development 
of  a  50  kilo- watt  Lundell  generator,  and  Fig.  74  shows  the 
distribution  of  flux  along  the  line  xy  of  Fig.  73.  The 
dotted  line  represents  the  distribution  at  no  load,  and  the 
heavy  line  the  distribution  at  full  load.  This  small  dis- 
torting effect  of  the  cross  turns  permits  the  employment  of 
a  small  air  gap  without  serious  sparking. 


88 


DYNAMO  ELECTRIC   MACHINERY. 


Fig.  73. 


OPERATION   OF   ARMATURES.  89 

Ryan  compensates  for  the  magnetizing  effects  of  the 
armature  winding  by  surrounding  the  armature  with  a  sta- 
tionary winding,  which  passes  through  perforations  in  the 
pole  faces.  These  stationary  windings  carry  the  whole 
current  of  the  machine.  This  method  prevents  all  spark- 
ing due  to  the  distortion  of  the  field,  but  it  does  not  pre- 
vent the  sparking  which  is  due  to  self-induction  and  mutual 
induction  of  the  armature  coils.  The  latter  sparking  is 
prevented  to  a  certain  extent  by  inserting  a  lug  between 
the  pole  horns,  which  is  magnetized  by  a  few  series  turns. 


Pig.  74. 

56.  Energy  Losses  in  Operation.  —  Besides  the  energy 
expended  in  exciting  the  field  coils,  there  are  losses  of 
energy  in  the  armature  and  connections,  as  follows  :  — 

a.  The  bearing  friction  and  the  windage.     This  loss  is 
generally  considered  independent  of  load,  but  it  is  ques- 
tionable whether  the  friction  does  not  increase  somewhat 
under  loads.     This  loss  is  from   1 5  per  cent  to  40  per  cent 
of  the  total  loss. 

b.  The  hysteresis  loss  in  the  iron  of  the  core  due  to  the 
continued  reversal  of  the  direction  of  magnetism  therein. 
According  to  Steinmetz's  Law,  the  hysteresis  loss  in  watts, 

h  = 


90  DYNAMO    ELECTRIC   MACHINERY. 

where  Fis  the  volume  of  iron  in  cubic  centimeters,  &  the 
flux  density,  n  the  number  of  magnetic  reversals  per  sec- 
ond, and  rj  a  constant  depending  in  value  upon  the  char- 
acter of  the  iron.  A  table  of  values  is  given  on  page  29. 
The  value  of  &  varies  at  different  loads  and  at  different 
places,  as  was  shown  by  Goldsborough,  so  this  loss  cannot 
be  said  to  be  proportional  to  the  speed  or  any  power  of 
the  voltage.  The  hysteresis  loss  is  from  1 5  per  cent  to 
40  per  cent  of  the  total  losses. 

c.  Eddy  currents  in  the  iron  and  the  copper  conductors. 
These  might  be   expected  to  vary  as  the  square  of  the 
speed,  but  they  do  not  for  the  same  reason  as  in  b.     Be- 
cause of    the  laminated    structure  of    the  core,   and   the 
slight  angular  breadth  of  the  conductors,  this  eddy  loss  is 
of  small  magnitude,  from  2  per  cent  to  10  per  cent  of  the 
losses.     It  may  amount  to  50  per  cent  of  the  losses  in 
the  case  of  smooth-core  armatures.     Eddy  currents  in  the 
pole  faces,  which  may  be  due  to  any  variation  in  the  re- 
luctance  encountered   by  the  lines    passing  through  the 
poles,  are  reduced  by  an  increase  of  air-gap  length.     They 
are  greatest  with  armature  cores  having  slots  with  large 
openings  at  the  top,  and  least  with  armatures  whose  in- 
ductors are  threaded  through  inclosed  channels  in  the  core. 

d.  The  armature  resistance  loss.    This  equals  I^R,  where 
/  is  the  total  current  of  the  machine,  and  R  the  resistance 
of  the  armature  measured  between  points  rubbed  by  the 
brushes  which   are   drawing  the  current  /.     This  is  ex- 
clusive of  the  transition  resistance  at  the  brushes.      In 
500  K.  w.  machines  the  I2R  loss  is  about  2  per  cent  of  the 
total  output.     In   5  K.  w.  machines  about  4  per  cent,  and 
in  smaller  machines  much  greater. 

e.  The  friction  of  the  brushes  against  the  commutator. 


OPERATION    OF   ARMATURES.  91 

This  loss  varies  about  as  the  speed,  and  its  importance  is 
generally  underestimated.  Carbon  brushes  press  upon  the 
commutator  with  a  force  of  from  I  to  12  pounds  per 
square  inch  of  contact.  Railway  motors  and  similar  ma- 
chines have  the  larger  value,  while  central-station  gene- 
rators have  the  smaller.  The  coefficient  of  friction  between 
carbon  and  copper  varies  from  0.28  to  0.32. 

f.  The  resistance  of  the  brushes  and  the  transition  re- 
sistance of  the  brush  contacts.  The  first  loss  varies  as 
the  square  of  the  current,  and  is  of  considerable  magni- 
tude in  low-potential  machines.  The  transition  resistance 
seems  to  vary  inversely  as  the  current,  thereby  always 
causing  a  constant  drop  of  voltage  amounting  to  from  i 
to  1.5  volts  per  transition. 

The  heat  produced  by  losses  b,  c,  and  d,  being  in  the 
armature  itself,  must  be  dissipated  by  the  conduction,  con- 
vection, and  radiation.  Experience  shows  that  from  2  to 
2.\  watts  can  be  radiated  from  every  square  inch  of  arma- 
ture surface  without  causing  a  dangerous  rise  of  tempera- 
ture in  the  armature  core.  It  is  found  that  about  500 
circular  mils  per  ampere  in  the  armature  conductors  brings 
the  loss  d  to  such  a  point  that,  added  to  the  losses  c  and 
by  they  together  give  about  2  watts  per  square  inch  of 
armature  surface  ;  hence  this  value  of  500  circular  mils  per 
ampere  is  the  mean  of  what  is  usually  adhered  to  in  winding 
armatures  of  commercial  machines. 


92  DYNAMO   ELECTRIC  MACHINERY. 


CHAPTER   VI. 

EFFICIENCY   OF   OPERATION. 

57.  Efficiency.  —  The  following  definition  and  discussion 
of  efficiency  is  taken  from  the  report  of  the  committee  on 
standardization  of  the  American  Institute  of  Electrical  En- 
gineers :  — 

The  "efficiency"  of  an  apparatus  is  the  ratio  of  its  net 
power  output  to  its  gross  power  input. 

Electric  power  should  be  measured  at  the  terminals  of 
the  apparatus. 

Mechanical  power  in  machines  should  be  measured  at 
the  pulley,  gearing,  coupling,  etc.,  thus  excluding  the  loss 
of  power  in  said  pulley,  gearing,  or  coupling,  but  including 
the  bearing  friction  and  windage.  The  magnitude  of  bear- 
ing friction  and  windage  may  be  considered  as  independent 
of  the  load.  The  loss  of  power  in  the  belt,  and  the  in- 
crease of  bearing  friction  due  to  belt  tension,  should  be 
excluded.  Where,  however,  a  machine  is  mounted  upon 
the  shaft  of  a  prime  mover,  in  such  a  manner  that  it  cannot 
be  separated  therefrom,  the  frictional  losses  in  bearings 
and  in  windage  which  ought,  by  definition,  to  be  included  in 
determining  the  efficiency,  should  be  excluded,  owing  to  the 
practical  impossibility  of  determining  them  satisfactorily. 
The  brush  friction,  however,  should  be  included. 

Where  a  machine  has  auxiliary  apparatus,  such  as  an  ex- 
citer, the  power  lost  in  the  auxiliary  apparatus  should  not 


EFFICIENCY   OF    OPERATION.  93 

be  charged  to  the  machine,  but  to  the  plant  consisting  of 
machine  and  auxiliary  apparatus  taken  together.  The 
plant  efficiency  in  such  cases  should  be  distinguished  from 
the  machine  efficiency. 

The  efficiency  may  be  determined  by  measuring  all  the 
losses  individually,  and  adding  their  sum  to  the  output  to 
derive  the  input,  or  subtracting  their  sum  from  the  input 
to  derive  the  output.  All  losses  should  be  measured  at,  or 
reduced  to,  the  temperature  assumed  in  continuous  opera- 
tion, or  in  operation  under  conditions  specified. 

58.  Coefficient  of  Conversion.  —  This  has  sometimes 
been  called  the  efficiency  of  conversion,  but  because  of  the 
definition  of  the  last  paragraph  it  is  better  not  to  use  the 
word  efficiency.  The  coefficient  of  conversion  ft  is  the  ratio 
of  the  total  electrical  energy  developed  in  the  armature 
winding  to  the  total  mechanical  energy  expended. 


where  P  is  the  power  expended  in  watts,  /<  the  armature 
current  in  amperes,  and  Et  the  E.M.F.  in  volts,  developed  in 
the  armature,  ft  is  always  less  than  unity,  because  of  the 
friction  and  windage  of  the  armature,  because  of  the  eddy 
currents  in  the  core  and  conductors,  and  because  of  the 
hysteresis  of  the  core. 

59.  Economic  Coefficient.  —  (rj)  This  coefficient  is  equal  to 
the  ratio  of  the  useful  electrical  energy  to  the  total  electri- 
cal energy  developed  in  the  armature  circuit.  It  is  always 
less  than  unity  because  of  the  necessary  loss  of  energy  in 
the  exciting  coils  and  in  the  armature  coils.  In  the  case 
of  a  series  dynamo,  if  we  let 


94  DYNAMO   ELECTRIC   MACHINERY. 

Et—  E.M.F.  generated  in  volts,  and 
E  =  Terminal  pressure  in  volts,  then  the  economic  coef- 
ficient for  a  current  of  /  amperes  is 

IE  _  E 

^-JE,-^: 

For  shunt  dynamos, 

IE 


Where  /is  the  current  in  the  outside  circuit,  /^the  current 
in  the  field  coils,  E  the  pressure  at  the  terminals  of  the 
machine,  and  Et  the  total  pressure  generated. 

The  efficiency  of  a  machine  e  is  evidently  the  product  of 
ft  and  rj. 

For  a  series  machine,  /  =  /(  and 

ftEt         E        IE 
£  =  /^=^r    X£t   =  -p' 

For  a  shunt  machine,  /  +  If—  It)  and 


=     ,  =  = 

-- 


P          (I+If)Et~  P 

Hence  the  product  of  (3  and  77  for  either  machine  is  the 
same,  and  corresponds  to  the  definition  of  efficiency. 

60.  Separately  Excited  Dynamos.  —  At  a  constant  speed 
and  constant  exciting  current  a  nearly  constant  total  pres- 
sure (Et)  is  generated  ;  and  it  is  almost  equal  to  the  pressure 
at  the  terminals  at  no  load  —  that  is,  on  open  circuit.  This 
follows  from  the  equation  for  the  average  pressure, 


where  —  -  is  the  number  of  revolutions  per  second,  5  the 
oo 

number  of  inductors,  <j>  the  flux  per  pair  of  poles,  and  /  the 


EFFICIENCY   OF   OPERATION. 


95 


number  of  pairs  of  poles.  If  the  speed  be  varied,  the  pres- 
sure will  vary  proportionally  if  no  load  is  on  the  machine. 
If,  however,  a  current  be  taken  off,  then  the  demagnetizing 
effects  of  the  armature  currents  become  evident  in  a 
change  of  the  value  of  <£,  and  there  will  be  a  falling  off  of 
pressure.  The  amount  of  this  deviation  is  dependent  upon 
the  composition  and  saturation  of  the  magnetic  circuit. 


100 


30  40 

AMPERES 

Fig.  75- 


This  effect  is  clearly  seen  in  the  curve  in  Fig.  75,  where 
the  armature  currents  are  measured  in  the  X  direction,  and 
the  pressure  in  the  Y  direction,  the  conditions  of  speed  and 
exciting  current  remaining  constant. 

Let  Et  =  the  total  volts  produced, 

E  —  the  volts  at  the  terminals  of  the  machine, 
Ra  =  the  resistance  of  the  armature, 
R  =  the  resistance  of  the  external  circuit,  and 
/  =  the  current  under  these  conditions. 


96  DYNAMO   ELECTRIC  MACHINERY. 

Then  for  a  separately  excited  machine, 


E  =  IR,  and 

El  PR  R 


and 


In  determining  the  efficiency  of  a  separately  excited 
machine  the  energy  lost  in  the  exciting  coils  must  be 
charged  against  the  coefficient  of  conversion. 

The  operation  of  any  dynamo  can  best  be  studied  by  in- 
spection of  a  curve  which  shows  the  relation  existing  be- 
tween the  current  generated  or  supplied  by  the  machine, 
and  the  voltage  under  which  it  operated.  Such  curves 
are  called  Characteristic  Curves  >  and  they  are  generally 
plotted  with  currents  for  abscissae  and  volts  for  ordinates. 
The  characteristic  curve  for  a  separately  excited  dynamo 
is  that  shown  in  Fig.  75. 

61.  Magnetos.  —  A  separately  excited  dynamo  whose 
field  is  maintained  by  a  permanent  magnet,  instead  of  an 
electric  magnet,  is  called  a  magneto.  These  machines 
from  their  similarity,  both  theoretically  and  practically, 
should  be  mentioned  together.  Magnetos  are,  however, 
generally  alternating  current  machines  with  slip  rings  in- 
stead of  commutators.  They  are  used  in  very  great  num- 
bers in  telephone  subscribers'  sets,  and  in  many  electrical 
businesses  for  testing  out  the  continuity  of  concealed  con- 
ductors, and  in  some  cases  for  determining  defective 
insulation.  To  the  armature  is  affixed  a  pinion,  meshing 
with  a  gear  turned  by  hand.  The  alternating  current  pro- 


EFFICIENCY   OF  OPERATION. 


97 


duced  is  passed  through  the  circuit  whose  continuity  it  is 
desired  to  determine,  and  then  passes  through  a  polarized 
bell  which  is  caused  to  ring 
These  machines  are  manufac- 
tured  so  as  to  ring  through 
an  external  resistance  of   as 
high  as  50,000  ohms  without 
undue  effort   at    the  handle. 
The  cut  Fig.  76  shows  a  com- 
mercial belt-driven  magneto. 

62.   Series  Dynamos.— 
Letting  E,  E9  R,  Ra,  and  7 
have  the  same  significance  as 
before,  represent    by  Rf  the 
resistance  of  the  field  winding,  and  by  Rb  the  resistance 
of  the  brushes  and  transition  contacts.     Then 


Fig.  76. 


E  =  IR, 

Et  = 


Rf  +  Ra 


whence  it  follows 
El 


*  -  EJ- 


The  value  of  rj  increases  as  Rm  Rb,  and  Rf  approach  zero. 
Rb  is  liable  to  be  of  greater  importance  than  is  imagined. 
In  low-tension  machines  all  the  resistances  are  small,  and 
care  must  be  taken  that  Rb  does  not  unduly  increase  the 
denominator  of  the  expression  for  rj ;  in  other  words,  cop- 
per brushes  should  be  used  on  low-voltage  machines. 

The   value    of  rj  varies  as  R,   but  the  load  varies  in- 
versely as  R ;  hence  rj  is  a  maximum  when  the  load  is  a 


98 


DYNAMO  ELECTRIC  MACHINERY. 


minimum,  and  17  =  I   when   R  =  oo,  or  there   is   no   load. 
Fig.  77  is  a  curve  showing  relation  between  17  and  load. 


1.0 


LOAD 

Fig.  77- 

63.  Characteristic  Curve  of  a  Series  Machine.  —  Fig. 
78  shows  the  curves  of  a  series  dynamo.  The  curve  of 
total  volts  Et  is  very  similar  to  the  magnetization  of  a 

magnetic  circuit  made 
up  of  iron  chiefly.  It 
falls  below  such  a 
curve  (a)  because 
saturation  causes  in- 
creased magnetic  leak- 
age, and  hence  the 
value  of  <f>  in  the 


equation^,  = 


1OAD 

Fig.  78. 


"      is  not  proportional  to 

the  total  flux,  and 
(b)  because  of  the  demagnetizing  and  cross  magnetizing 
effects  of  the  armature  currents.  The  curve  E,  starts 
above  zero  because  of  the  residual  magnetism  in  the  cores 
of  the  field  magnets.  If  operated  under  constant  load,  a 


EFFICIENCY   OF   OPERATION.  99 

series  dynamo  will  give  E,  directly  proportional  to  the 
speed. 

The  straight  line  represents  the  loss  or  drop  of  potential 
due  to  the  resistances  of  the  machine,  Ra,  Rb,  and  Rj. 
Since  drop  of  potential  is  proportional  to  the  resistance, 
this  is  a  straight  line,  and  must  pass  through  the  origin. 
This  loss  line  can  be  established  by  a  point  found  by  as- 
suming the  lost  volts  EI  and  solving  for  the  current  I  from 
the  equation  I  (Ra  +  R6  +  R7)  =  Ej.  For  example,  if  the 
resistances  Ra  +  Rj+  R/be  assumed  as  0.2  ohrn,  then  10 
volts  would  be  lost  in  them  only  when  50  amperes  were 
flowing  through  them.  A  line  drawn  through  the  origin, 
and  a  point  on  the  characteristic  curve  diagram  whose 
coordinates  were  10  volts  and  50  amperes,  would  at  every 
point  give  the  volts  lost  in  sending  the  corresponding  num- 
ber of  amperes. 

The  curve  E  showing  the  E.M.F.  at  the  terminals  of 
the  machine  as  a  function  of  the  current  output  is  found 
by  subtracting  the  ordinates  of  the  loss  line  from  those  of 
Et  and  using  the  differences  as  the  ordinates  of  E.  In 
practice  Et  cannot  be  directly  found  ;  but  the  terminal  volts 
and  the  current  can  be  measured,  thus  giving  the  curve  E, 
and  from  a  knowledge  of  the  loss  line  the  curve  Et  can  be 
derived. 

The  operation  of  some  special  forms  of  series  machines 
will  be  discussed  in  the  chapter  on  arc-lighting  machines. 

64.  Power  Lines.  —  Where  volts  and  amperes  are  used 
as  ordinates  and  abscissae,  lines  can  be  drawn  connecting 
points  of  constant  product  of  the  two,  representing  watts 
or  power.  Fig.  79  shows  such  lines  drawn  for  one,  two, 
and  three  kilowatts.  If  E  be  the  external  characteristic  of 


100 


DYNAMO  ELECTRIC  MACHINERY. 


a  dynamo,  then  the  curves  make  it  apparent  that  the  ma- 
chine cannot   generate   3  K.W.,  but   that  for  most  values 

under  3  K.  w. 
there  will  be  two 
loads  under  which 
the  generator  can 
run  and  yield  the 
same  voltage. 


65.   Shunt  Dy- 
namos.  —  In 

shunt-wound  ma- 
chines the  cur- 
rent in  the  arma- 
ture is  the  sum  of 
the  current  in  the 
field  coils  and  of 
that  in  the  ex- 
ternal circuit,  or 


20  30 

AMPERES 


Fig.  79- 


I  a  =  If  +  /•     For  sake  of  simplicity  we  will  assume  7a  =  /. 

Practically  this  introduces  but  a  small  error  under  ordinary 

conditions  of  load. 

E* 
R 


IE, 


IjE 


+  i* 


, 


R 


+ 


R 


A  -L\-n  A  -»«-«  •*«• 

~f?  +  ~&  +  J?          *  +~J?+~K 
J\.         J\          J\.j  J\.         J\.f 

To   determine    what  value  of  R  will  enable  a  given   ma- 
chine to  operate  with  a  maximum  economic  coefficient  — 


EFFICIENCY   OF   OPERATION.  .  ,„  ,    ,,  Ipl, 


place  the  differential  coefficient  of  rj,  in  respect  to  R  con- 
sidered as  a  variable,  equal  to  o  and  solve  for  R 


dR 


The  external  resistance  must  be  a  mean  proportional  be- 
tween Ra  and  Rfy  and  the  maximum  economic  coefficient  is 


i  +  2V^ 

66.  Characteristic  Curve  of  a  Shunt  Dynamo.  —  In  Fig. 
80  the  curve  E  is  plotted  from  experimental  results  obtained 
while  the  machine  is  running  at  various  loads.  To  get  satis- 


factory  results,  one  should  begin  with  an  infinite  resistance 
in  the  external  circuit,  which  is  then  reduced  step  by  step. 
In  some  small  machines  it  can  be  reduced  to  zero  without  an 
extreme  elevation  of  temperature  due  to  excessive  currents. 
As  a  rule,  only  the  upper  and  lower  values  of  E,  correspond- 
ing to  currents  between  o  and  a  definite  maximum  value,  can 


102  '    DYNAMO  ELECTRIC    MACHINERY. 


be  obtained.  The  loss  line  L  is  obtained  by  calculation  as 
before  in  the  case  of  the  series  machine.  The  curve 
showing  the  relation  between  external  current  and  total 
volts,  Et>  is  obtained  by  adding  the  ordinates  of  L  to  those 
of  E.  The  drop  in  E  is  at  first  due  chiefly  to  the  drop  re- 
sulting from  armature  resistance.  As  the  current  increases, 
the  effects  of  armature  reaction  and  saturation  of  the 
magnetic  circuit  become  evident.  At  the  same  time  E  is 
affected  by  a  decrease  of  the  shunt-field  current  due  to 
the  fall  of  potential  at  the  terminals  of  the  field  circuit. 
This  soon  becomes  the  predominating  cause  of  drop,  and  to 
such  an  extent  that  the  curve  turns  back  toward  the  origin. 
When  zero  resistance  is  in  the  external  circuit,  of  course  no 
current  flows  through  the  field,  and  the  few  volts  then 
produced  are  due  to  residual  magnetism.  It  must  be 
remembered  that  while  E  is  a  double-valued  function  of  / 
it  is  a  single-valued  function  of  R. 

The  voltage  of  a  shunt  machine  generally  increases  more 
rapidly  than  the  speed.  An  increase  of  speed  not  only  in- 
creases primarily  the  number  of  volts  generated,  but  also 
increases  the  armature  flux  <£  because  of  increased  excita- 
tion. The  condition  of  the  magnetic  circuit  as  regards 
saturation  determines  whether  this  secondary  influence 
shall  be  great  or  small. 


CONSTANT   POTENTIAL  DYNAMOS.  103 


CHAPTER   VIL 

CONSTANT   POTENTIAL  DYNAMOS. 

67.  Constant  Potential  Supply The  method  of  sup- 
plying, at  any  point  of  usage,  current  at  a  constant  poten- 
tial irrespective  of  the  load  which  is  there  or  elsewhere,  is 
used  in  the  distribution  of  electrical  energy  for  purposes 
of  incandescent  electric  lighting,  for  consumption  in  con- 
stant pressure  motors,  and  for  trolley-car  propulsion.     The 
great  sensitiveness  of   the  candle   power  of    incandescent 
lamps  to  a  change  in  voltage,  the  candle  power  varying 
as   the   fourth    power    or  more    of    the    voltage,    requires 
that  the  pressure  in  lines  used  for  lighting  must  not  vary 
by  more  than  3  per  cent  of  its  rated  value.     In  street-car 
work,  where  the  load  suffers   tremendous  variations,  con- 
stant potential  supply  is  equally  as  imperative  for  satisfac- 
tory operation. 

68.  Methods   of   Obtaining    Constant    Potential For 

accomplishing  this  result  many  devices  have  been    tried, 
the  more  important  of  which  are  :  — 

a  Automatic  variation  of  the  resistance  in  the  field  cir- 
cuit of  shunt  machines. 

b  Automatic  change  of  the  position  of  the  brushes  and 
commutating  plane. 

c  Automatic  variation  of  armature  speed. 


IO4 


DYNAMO   ELECTRIC   MACHINERY. 


d  Hand  regulation  of  a  resistance  in  series  with  a  shunt 
field  coil. 

e  Self-regulation. 

Of  these  the  first  three  methods  are  no  longer  employed, 
and  either  hand  regulation  or  self-regulation  or  both  to- 
gether are  relied  upon  to  maintain  the  constant  voltage 
under  varying  loads. 

69.   Hand  Regulation Inspection  of  the  characteristic 

curves  of  either  the  shunt  or  the  separately  excited  dynamo 
shows  a  drop  in  the  voltage  as  the  load  increases.  This  is 
due  to  the  internal  resistance  of  the  armature  and  the 
demagnetizing  effect  of  armature  reaction.  In  the  formula 

r-  i    jr. 

for  the  E.M.F.  of  a  machine,  E  =  ,  the  only  quan- 

tity that   is  practical   to  vary  is  </>.     This   can  easily  be 

accomplished  by  regulating 
the  amount  of  resistance  in 
a  rheostat,  which  is  in  series 
with  the  field  coils  and  which 
therefore  governs  the  amount 
of  current  in  them,  as  in 
Fig.  81. 

.  In  distributing  current  for 
use  among  a  number  of  con- 
sumers the  current  is  carried 
to  feeding-points  which  are 
near  the  locality  they  supply,  but  may  be  distant  from  the 
station.  It  is  desirable  to  keep  the  pressure  at  these 
points  at  a  constant  value,  irrespective  of  the  varying  loss 
of  potential  that  is  going  on  because  of  the  resistance  of 
the  conductors  leading  to  them.  To  achieve  this  end  the 


CONSTANT   POTENTIAL  DYNAMOS. 


105 


Edison  system  employs  feeders  to  carry  the  current  to  the 
feeding-points.  Each  feeder  is  accompanied  by  a  pilot  wire 
imbedded  in  the  insulation.  At  the  feeding-point  the  pilot 
wires  are  attached  to  the  feeder  terminals,  and  at  the  sta- 
tion end  are  attached  to  a  voltmeter,  so  that  one  can,  in 
the  station,  regulate  the  pressure  not  at  the  machine  ter- 
minals but  at  the  distant  distributing  point. 

70.   Field  Rheostats For  varying  the  current  in  the 

shunt  fields  of  dynamos,  it  is  usual  to  employ  field  rheostats 


"L. 

*MiiiiiniiiriunnH 


Fig.  82. 


which  are  mounted  on  the  switch-board  along  with  indicat- 
ing instruments.  A  form  of  such  rheostatic  regulators  is 
the  so-called  Packed  Card  Rheostat,  manufactured  by  the 
General  Electric  Company.  This  derives  its  name  from 


io6 


DYNAMO   ELECTRIC   MACHINERY. 


the  method  of  constructing  it.  A  tube  of  asbestos,  in- 
closing a  steel  mandrel,  is  wound  with  a  chosen  amount  of 
German-silver  wire  or  ribbon.-  The  tube  is  then  removed 
from  the  mandrel,  and  pressed  into  the  form  of  cards  as 
shown  in  Fig.  82.  These  cards  are  then  assembled,  with 
interposed  asbestos,  in  sufficient  numbers  to  make  up  the 
required  resistance  of  the  rheostat.  Iron  plates,  somewhat 


Fig.  83. 

wider  than  the  cards,  are  introduced  at  intervals,  and  thus 
increase  the  radiating  surface.  The  whole  is  held  together 
by  iron  end  plates  and  bolts,  as  shown  in  Fig.  83.  Con- 
tact bolts  are  connected  with  various  points  of  the  conduc- 
ting part  of  the  rheostat,  and  these  bolts  are  connected 
through  a  wiping-finger  with  the  field  circuit.  Fig.  84  shows 
a  rheostat  of  this  type  built  for  regulating  a  railway  gene- 
rator and  arranged  to  be  placed  on  the  back  of  a  switch- 


CONSTANT   POTENTIAL  DYNAMOS. 


ID/ 


board  with  the  regulating  handle  projecting  in  front. 
For  the  largest  generators  resistances  made  of  iron  grids 
supported  in  iron  frames  are  employed.  Both  of  these 
constructions  are  fire-proof  and  easily  repaired  in  case  of 
accident. 


When  large  generators,  such  as  are  used  in  railroad  work, 
have  their  field  circuits  opened,  the  E.M.F.  self-induced 
by  the  disappearance  of  the  flux  in  the  fields  is  liable  to 
reach  such  a  magnitude  as  to  pierce  the  insulation  of  the 
field  coils  and  destroy  their  usefulness.  To  obviate  this, 
before  the  field  circuit  is  broken,  the  field  coils  are  con- 
nected (Fig.  85)  through  a  high  discharge  resistance,  and 
the  current  in  them  is  allowed  to  die  out  slowly.  It  is 
thus  unattended  with  any  destructive  potential  differences. 


io8 


DYNAMO   ELECTRIC   MACHINERY. 


The  Edison  Electric  Illuminating  Company  of  New  York 
City,  in  the  case  of  its  Duane-street  generators,  allows  the 
field  circuits  to  discharge  themselves  through  an  arc  light. 
Another  form  of  field  rheostat  is  the  Carpenter  Enamel 
Rheostat,  made  by  the  Ward  Leonard  Electric  Company. 
In  this  rheostat  the  heat  generated  is  not  radiated  directly 


Binding  Posts 


Parallel  Resistance" 

Rheostat  Switch 


Pilot"  Lamp  Oc= 


Tietd 


/Irmatur-e 


Fig.  85. 


from  the  surface  of  the  wire,  but  is  conducted  to  a  sup- 
porting plate,  which  then  becomes  the  radiating  surface. 
The  resistance  wires  are  surrounded  with  an  enamel,  which 
attaches  them  to  the  supporting  plates,  insulates  them 
therefrom,  and  protects  them  from  corrosion.  Owing  to 
the  increased  radiating  surface  thus  obtained,  a  shorter  and 
smaller  wire  can  be  used  for  a  given  volt-ampere  capacity 


CONSTANT   POTENTIAL   DYNAMOS. 


ICQ 


than  if  the  wire  were  merely  exposed  to  the  air.     No  con- 
sideration of  the  mechanical  strength  of  the  wire  enters 


Fig.  86. 


into  the  design  of  this  resistance,  since  it  is  supported  and 
protected  by  the  enamel.     To  further  increase  the  radiat- 


Fig.  87. 

ing  surface,  the  back  of  the  plate  is  provided  with  raised 
annular  ribs.   The'makers  claim  that  this  rheostat  can  radi- 


no 


DYNAMO  ELECTRIC   MACHINERY. 


ate  5  watts  for  each  square  inch  of  one  surface.     Thus  a 
plate    10  by    10   inches   will   dissipate    500    watts.      The 

method  of  using  iron  radiat- 
ing plates  for  purposes  of 
dissipating  large  amounts 
of  heat  is  to  be  found  in 
the  rheostats  of  many  man- 
ufacturers. Wirt  (Fig.  88) 
incloses  resistance  wire  or 
ribbon  in  radiating  plates, 
insulating  them  from  each 
other  by  means  of  mica. 
Other  firms  employ  sand  as 
an  insulating  material. 

7 1 .  Self -Regulation .  - 
By  far  the  most  elegant 
method  of  constant  poten- 
tial regulation  is  that  in 
which  the  main  current  of 
the  machine  is  utilized  in 
maintaining  constant  the  magnetic  flux  <£  through  the 
armature.  This  is  accomplished  by  passing  all  or  the 
greater  part  of  the  current  produced  in  the  armature  a 
few  times  around  the  field  magnets,  so  that  an  increased 
load  on  the  armature  increases  the  magnetizing  ampere 
turns  of  the  field  coils.  These  series  turns,  when  rightly 
proportioned,  can  be  made  to  compensate  for  a  part,  for 
all,  or  for  even  more  than  all  of  the  drop.  This  device 
can  be  used  in  connection  with  any  other  form  of 
excitation,  as  permanent  magnets,  separate  excitation, 
or  shunt  excitation.  In  the  last  case,  the  dynamo  is 


Fig.  88. 


CONSTANT   POTENTIAL   DYNAMOS.  Ill 

said  to  be  compotmd  wound,  as  described  in  §  45.  If 
the  machine  is  designed  to  maintain  a  constant  pressure 
at  some  distant  feeding-point,  instead  of  at  the  machine 
terminals,  the  machine  is  said  to  be  over-compounded,  since 
the  potential  at  the  terminals  will  rise  on  increase  of  load. 
From  3  to  5  per  cent  over-compounding  is  frequent  in 
machines  used  to  supply  lighting  circuits,  and  10  per  cent 
over-compounding  is  usual  in  railway  generators. 

72.  Economic  Coefficient  of  a  Compound  Machine.  - 
To  discover  the  value  of  77  in  this  case,  let  R  be  the  resis- 
tance of  the  external  circuit,  Rs  the  resistance  of  the  series 
turns,  Rsi  the  resistance  of  the  shunt-field,  and  Ra  the 
resistance  of  the  armature.  Then  assuming  that  the  cur- 
rent in  the  armature  is  the  same  as  in  the  external  circuit, 
an  assumption  which  is  warranted  in  the  case  of  commer- 
cial machines, 


Considering  R  as  a  variable   dependent  on  77,  and  solving 
for  a  maximum  of  77 


.  -,-.  +     +_  0 

dR  R        &^  & 


Hence  it  is  seen  that  the  maximum  economic  coefficient  is 
obtained,  when  the  external  resistance  is  the  geometric 


I  12 


DYNAMO   ELECTRIC   MACHINERY. 


mean  between  the  shunt-field  resistance  and  the  sum  of 
the  resistances  of  the  series  field  and  of  the  armature. 
Under  these  conditions, 


i  4-2 


73.  Efficiency  of  Compound  Machines  --  The  efficiency 
of  a  generator  increases  with  the  size,  being  quite  low  on 
small  machines,  and  sometimes  very  high  on  the  larger 
dynamos.  Since  the  distribution  of  the  magnetic  and  elec- 
trical losses  of  a  generator  lies  within  the  discretion  of  the 
designer,  it  is  possible  to  so  design  a  machine  as  to  have 

its  point  of  maximum  effi- 
ciency at  full  load  or  at  a 
smaller  load,  for  instance, 
at  one-fourth  load.  The 
two  following  cuts  show 
the  relations  between  effi- 
ciencies and  loads  on  two 
different  machines. 


100 

44- 

90 
80 
70 
60 
50 
40 
30 
20 
10 
0 

-ft 

X 

/ 

/ 

M 

/ 

1 

y 

III         EFFICIENCY  CURVE 
200  K.WV    SIZE  224 
DIRECT  DYNAMO  ~  \ 

JM     CROCKPE  WHEELED    ELEc'TBlc  CO 

- 

o- 

t 

( 

-PUT 

Kl 

0- 

WA 

FT, 

4- 

04 

-    0 


74.  The  Compounding 
Rectifier  ---  The  gradual 
saturation  of  the  fields  of 
a  generator  as  full  load 
approaches  causes  the 
E.M.F.  of  even  a  com- 


80    ^20   160   ^00  m  £ 
Fig.  89. 

pound-wound  machine  to  sag  at  full  load,  or  if  the  machine 
is  so  heavily  compounded  that  it  maintains  its  potential  at 


CONSTANT   POTENTIAL   DYNAMOS. 


full  load,  its  voltage  will  rise  abnormally  at  some  load  less 
than  full  load.  To  counteract  this  effect,  the  Crocker 
Wheeler  Company  employs  a  device  which  is  termed  a 
compounding  rectifier.  It  consists  of  a  suitable  resistance 
shunted  across  the  ter- 
minals of  the  series  field  100f 
coils.  The  full  armature 
current  therefore  divides 
between  this  rectifying 
coil  and  the  series  coils. 
As  the  load  increases, 
more  current  passes 
through  each,  but  the 
coils  are  so  designed 
that  this  increase  heats 
the  rectifier  and  causes 
its  resistance  to  increase, 
while  the  resistance  of 
the  series  coils  remains 
practically  unaltered. 

Thus,  as  the  load  increases,  a  larger  proportion  of  the  whole 
current  passes  through  the  series  coils,  and  this  compen- 
sates for  the  sag  in  voltage  that  would  otherwise  have 
existed. 

70.  Theory  of  Self -Regulation.  —  To  determine  the 
number  of  turns  of  wire  necessary  to  be  used  in  the  series 
regulating  coils  which  are  wound  on  the  field  magnets  of  a 
compound  machine, 

Let  n  =  number  of  shunt  turns. 
n'  =  number  of  series  turns. 
B  =  number  of  back  turns. 


20     40 


80    100  120    140  160 
Pig.  90. 


114  DYNAMO    ELECTRIC   MACHINERY. 

X=  number  of  cross  turns. 
J?a  +  s  =  the  resistance  of  the  armature   plus  that  of   the 

series  coil. 

fsh  =  current  in  the  shunt  coils. 
I  =  current  in  the  armature  and  also  in  the  series  coils, 

since  they  are  practically  the  same. 
Et  =  total  pressure  developed. 
E  =  pressure  at  terminals. 
A.  =  the  coefficient  of  magnetic  leakage. 
(R  =  the  reluctance  of  magnetic  circuit  when  armature 

is  idle.    Then 


(H—  -  =  reluctance  with  current  /in  armature. 

nlsh 

Let  <£,  <£',  <£",  =  flux  in  the  armature  under  different  con- 
ditions of  working. 

When  no  current  flows  in  the  armature, 


,=*=     2C 

io<RX         '  5  fltt" 

When  the  current  /  flows  in  the  armature, 


(RX     -X71  H- 


/  

where  -=  ^- Jl? — f?,  hence  rt:  represents  the  ratio  of  the 

a  nSsh 

reluctance  at  no  load  to  the  reluctance  with  the  load  /. 
The  latter  value  is  the  greater  because  of  the  skewing 
effect  of  the  cross  turns,  a,  therefore,  is  less  than  i . 

The  flux  in  the  armature  which  is  due  to  the  shunt  coils 
only,  when  a  current  I  flows  in  the  armature  circuit,  is 


CONSTANT   POTENTIAL   DYNAMOS.  115 

Thus  under  load  the  amature  flux  due  to  the  shunt  coils  is 

decreased  in  the  ratio, 

<}>"  :  <j>  :  :  a  :  i. 

The  series  turns  must  make  up  this  loss,  and  also  compen- 
sate for  the  loss  due  to  the  back  turns  and  for  the  electri- 
cal losses  due  to  the  resistances  of  the  armature  and  the 
series  coils. 

Now, 
and 


For  convenience  let 
*  =  2 


The  first  term  of  the  right-hand  member  can  be  written 
knlsh  —  k  (i  —a)  nfsh,  in  which  the  expression  knl^  repre- 
sents the  total  voltage  developed  by  the  machine  at  no 
load,  which  is  therefore  the  terminal  voltage  at  that  load, 
or  in  other  words  is  the  voltage  for  which  the  machine  is 
to  be  compounded.  The  equation  for  the  terminal  voltage 
at  the  load  /  therefore  becomes 

E  =  knlsh  —  k  (i  —  a)  nlsh  +  \ka  (nr  —  B]  —  l?a  +  s]  I. 
Evidently,  if  E  is  to  equal  knlsh  at  any  and  every  load, 

-  k(i  -a)  nlsh  +  \ka  (n'  -  B)  -  £a+s]  /=  o, 
whence 

n  =~a       7"  ~*~       +    ka 


Il6  DYNAMO   ELECTRIC   MACHINERY. 

Remembering  that  -=  =  -g?  and  also  that  the  percentage 
of  electrical  energy  loss  in  the  field/  =  -j  100, 

'  l    ~  a  A,  R  _L   nI*R*  +  * 

n  = pn  4-  JB  -\ —        = —  • 


100  a 


In  this  value  for  n'  the  first  term  gives  the  number  of 
series  turns  required  to  overcome  the  skewing  due  to  the 
cross  turns ;  the  second  term  gives  the  series  turns  neces- 
sary to  compensate  for  the  armature  back  turns ;  and  the 
third  term  shows  the  number  of  series  turns  to  balance  the 
loss  due  to  the  resistances  of  the  armature  and  the  series 
coils. 

The  difficulty  of  applying  this  formula  lies  in  finding  a 
suitable  value  for  a.  This  differs  in  different  machines, 
having  according  to  Jackson  a  value  of  from  .75  to  .85  at 
full  load.  It  is  of  course  dependent  on  the  load,  and  has  a 
value  of  unity  for  no  load. 

76.  Views  of  the  American  Institute  of  Electrical 
Engineers.  —  The  following  statements  concerning  the 
regulation  of  direct  current  apparatus  are  taken  from  the 
report  of  the  Standardization  committee  of  the  Insti- 
tute :  — 

The  regulation  of  an  apparatus  intended  for  the  gene- 
ration of  constant  potential,  constant  current,  constant 
speed,  etc.,  is  to  be  measured  by  the  maximum  variation 
of  potential,  current,  speed,  etc.,  occurring  within  the 
range  from  full  load  to  no  load  under  such  constant  con- 
ditions of  operation  as  give  the  required  full-load  values, 
the  condition  of  full  load  being  considered  in  all  cases  as 
the  normal  condition  of  operation. 


CONSTANT   POTENTIAL  DYNAMOS.  117 

The  regulation  of  an  apparatus  intended  for  the  gene- 
ration of  a  potential,  current,  speed,  etc.,  varying  in  a  defi- 
nite manner  between  full  load  and  no  load,  is  to  be 
measured  by  the  maximum  variation  of  potential,  current, 
speed,  etc.,  from  the  satisfied  condition,  under  such  con- 
stant conditions  of  operation  as  give  the  required  full-load 
values. 

If  the  manner  in  which  the  variation  in  potential,  cur- 
rent, speed,  etc.,  between  full  load  and  no  load  is  not  speci- 
fied, it  should  be  assumed  to  be  a  simple  linear  relation ; 
i.  e.,  undergoing  uniform  variation  between  full  load  and  no 
load. 

The  regulation  of  an  apparatus  may,  therefore,  differ 
according  to  its  qualification  for  use.  Thus  the  regulation 
of  a  compound-wound  generator  specified  as  a  constant- 
potential  generator  will  be  different  from  that  it  possesses 
when  specified  as  an  over-compounded  generator. 

The  regulation  is  given  in  percentage  of  the  full-load 
value  of  potential,  current,  speed,  etc.  ;  and  the  apparatus 
should  be  steadily  operated  during  the  test  under  the  same 
conditions  as  at  full  load. 

The  regulation  of  generators  is  to  be  determined  at  con- 
stant speed. 

The  regulation  of  a  generator  unit,  consisting  of  a  gen- 
erator united  with  a  prime  mover,  should  be  determined  at 
constant  conditions  of  the  prime  mover ;  i.  e.,  constant 
steam  pressure,  head,  etc.  It  would  include  the  inherent 
speed  variations  of  the  prime  mover.  For  this  reason  the 
regulation  of  a  generator  unit  is  to  be  distinguished  from 
the  regulation  of  either  the  prime  mover  or  of  the  gene- 
rator contained  in  it,  when  taken  separately. 

In  commutating  machines  as  direct  current  generators 


Il8  DYNAMO   ELECTRIC   MACHINERY. 

and  motors,  the  regulation  is  to  be  determined  under  the 
following  conditions : 

a.  At  constant  excitation  in  separately  excited  fields, 

b.  With  constant  resistance  in  shunt-field  circuits,  and 

c.  With  constant  resistance  shunting  series  fields ;  i.e., 
the  field  adjustment  should  remain  constant,  and  should  be 
so  chosen  as  to  give  the  required  full-load  voltage  at  full- 
load  current. 

In  constant  potential  machines  the  regulation  is  the 
ratio  of  the  maximum  difference  of  terminal  voltage  from 
the  rated  full-load  value  (occurring  within  the  range  from 
full-load  to  open  circuit),  to  the  full-load  terminal  voltage. 

In  constant  current  machines  the  regulation  is  the  ratio 
of  the  maximum  difference  of  current  from  the  rated  full- 
load  value  (occurring  within  the  range  from  full  load  to 
short  circuit),  to  the  full-load  current. 

In  over-compounded  machines,  the  regulation  is  the 
ratio  of  the  maximum  difference  in  voltage  from  a  straight 
line  connecting  the  no-load  and  full-load  values  of  terminal 
voltage  as  function  of  the  current,  to  the  full-load  terminal 
voltage. 

77.  Direct  Driven  Light  Generators.  —  The  tendency  of 
modern  engineering  practice  is  to  install  lighting  gene- 
rators which  are  directly  connected  with  the  steam  engines 
which  drive  them.  Owing  to  the  inherent  speed  of  engines 
being  smaller  than  that  of  generators,  direct  connected 
armatures  are  designed  to  run  at  a  lower  speed  than  belt- 
driven  ones.  Economical  construction  demands  that  they 
be  of  the  multipolar  type.  They  require  less  floor  space 
per  kilowatt  than  the  belt-driven  machines ;  and  this  is  a 
question  of  considerable  importance  in  many  installations. 


CONSTANT   POTENTIAL   DYNAMOS. 


119 


They  have  a  higher  efficiency  of  operation  consequent 
upon  the  elimination  of  losses  in  belting  and  counter- 
shafting.  They  also  permit  of  operation  of  isolated  plants 
in  residences  and  other  places  where  the  noise  resulting 
from  belt-driven  machinery  would  not  be  tolerated. 

In  order  that  standard  generators  may  be  easily  con- 
nected with  engines  of  any  make,  and  vice  versa,  commit- 
tees from  the  American  Societies  of  Electrical  Engineers 
and  of  Mechanical  Engineers  have  recommended  the 
adoption  of  the  following  standard  sizes,  speeds,  and  arma- 
ture shaft  fits  :  — 


Sizes  in  K.  W.  Capacity  . 

5 

7-5 

10 

15 

20 

25 

35 

Speeds  in  Rev.  per  Minute 

45° 

425 

400 

375 

35° 

325 

310 

Armature  Fit  in  Inches    . 

3 

3 

3% 

3% 

4 

4 

4% 

Sizes  in  K.  W.  Capacity  . 

50 

75 

IOO 

125 

J5° 

200 

250 

300 

Speeds  in  Rev.  per  Min.  . 

290 

275 

250 

235 

220 

200 

190 

1  80 

Armature  Fit  in  Inches    . 

5 

6 

7 

7X 

8 

9 

10 

ii 

Fig.  91  shows  a  machine  made  by  the  Westinghouse 
Electric  Manufacturing  Company  in  standard  sizes  of  100, 
150,  200,  500,  and  675  K.  w.,  at  125  volts.  The  field 
frame  is  circular  and  divided  in  a  vertical  plane.  The  pole 
pieces  are  of  laminated  sheet  steel,  cast  into  the  frame. 
Projecting  from  the  field  frame  are  brackets,  which  hold 
and  carry  the  brush-holder  mechanism.  This  consists  of 
a  ring  concentric  with  the  axis  of  the  armature.  Upon 
its  rim  is  a  gear,  which  engages  with  a  worm  operated  by 
a  hand-wheel.  The  simultaneous  shifting  of  the  brush 
can  be  accomplished  by  the  turning  of  the  hand-wheel. 
The  slotted  armature  disks  are  made  of  sheet  steel,  and 


120 


DYNAMO   ELECTRIC   MACHINERY. 


are  held  together  by  cast-iron  end  plates.  The  disks  and 
end  plates  are  mounted  upon  a  cast-iron  spider,  which  also 
carries  the  commutator.  The  spider  is  fitted  so  as  to  be 
pressed  upon  the  engine  shaft  and  keyed  to  it.  The  con- 


Fig,  gi. 

ductors  are  bars  of  copper,  which  are  forged  into  shape  on 
cast-iron  formers  wound  and  insulated  with  mica  and  ful- 
lerboard. 

Figs.  92  and  93  represent  a  front  and  rear  view  of  a 


CONSTANT   POTENTIAL   DYNAMOS. 


121 


General  Electric  Company's  Form  L  generator.  The 
frame,  of  a  circular  form,  is  divided  in  a  horizontal  plane, 
and  is  made  of  soft  cast  iron.  To  it  are  bolted  pole  pieces 


Fig.  92. 

which  are  made  of  soft  cast  steel.  A  skeleton,  circular, 
disk-like  brush-holder  yoke  is  fastened  to  the  frame  by 
means  of  three  slots  and  bolts,  and  is  capable  of  sufficient 
angular  rotation  to  permit  of  the  proper  adjustment  of 


122 


DYNAMO   ELECTRIC   MACHINERY. 


the  brushes.  The  movement  is  accomplished  by  means  of 
a  hand-wheel  and  pinion.  The  armature  spider  is  so  con- 
structed that  it  receives  the  commutator  as  well  as  the 


Fig.  93. 


disks  and  the  armature  windings.  It  is  open  so  as  to  offer 
no  obstruction  to  the  free  and  thorough  circulation  of  air 
through  it,  which  permits  of  a  perfect  ventilation.  The 
windings  are  of  copper  bars,  and  the  end  connections  are 


CONSTANT   POTENTIAL   DYNAMOS.  123 

supported  by  flanges  which  protect  them  from  mechanical 
injury.  The  commutator  shell  is  pressed  upon  the  arma- 
ture spider. 


Fig.  94. 

The  Crocker  Wheeler  Electric  Company's  direct-con- 
nected and  belt-driven  generators  differ  from  others  which 
have  been  described,  chiefly  because  of  the  shape  of  the 
field-magnet  frame  and  the  method  of  armature  winding. 
The  field  frame  shown  in  Fig.  94  is  circular  in  form,  and  is 


124  DYNAMO   ELECTRIC   MACHINERY. 

divided  in  a  horizontal  plane.  These  frames  are  of  cast 
iron,  and  have  short  internal  flanges  on  each  side,  which 
mechanically  strengthen  the  frame,  and  offer  considerable 
protection  from  mechanical  injury  to  the  field  coils.  The 
round  poles  are  of  cast  steel,  cast-welded  into  the  frame. 
They  are  provided  with  removable  cast-iron  shoes,  which 
are  clamped  in  place  after  the  field  coils  have  been  put  on. 
The  armatures,  instead  of  being  bar-wound,  are  wound 


Fig.  95. 

with  solid  copper  wire  of  large  sizes,  which  are  triple 
cotton  covered.  The  conductors  are  threaded  through 
tubes  which  are  placed  one  upon  the  other,  and  which  are 
made  of  micanite  cloth  and  press-board  rolled  up  on  a 
form  and  glued  together.  The  brush  holders  and  brush 
rigging  were  shown  in  Figs.  48  and  49. 

The  Sprague  Electric  Company  manufactures  two  types 
of  Lundell  generators,  both  for  direct  connection  and  for 
belt  connection.  They  are,  namely,  the  split-pole  type, 
which  employs  the  principle  laid  down  in  paragraph  55 


CONSTANT   POTENTIAL   DYNAMOS. 


125 


for  compensating  for  armature  reaction,  and  the  single-coil 
type,  which  takes  its  name  from  the  peculiar  shape  of  the 
field  frame  and  poles,  which  permits  of  the  use  of  but  a 
single  field  coil.  Both  frames  are  of  the  circular  type, 


Fig.  96. 

the  split-pole  field  being  divided  in  a  horizontal  plane, 
and  the  single-coil  type  being  divided  in  a  vertical  plane 
which  is  perpendicular  to  the  axis  of  the  armature.  A 
split  pole,  with  its  windings,  is  shown  in  Fig.  95.  The 
compound  coil  is  placed  nearer  the  shoe  than  the  shunt 


126 


DYNAMO   ELECTRIC    MACHINERY. 


coil,  and  both  are  kept  in  place  by  lugs,  as  shown  in  the 
figure.     Fig.    96    shows   a    6-pole,    single-coil   type   field- 


magnet  frame  with  its  coil  inclosed  in  the  frame.  The 
brush  holders  which  are  employed  on  both  types  of  ma- 
chine are  illustrated  in  Fig.  97,  the  brushes  being  of 


CONSTANT   POTENTIAL  DYNAMOS. 


127 


carbon  used  radially,  and  being  perforated  to  receive  a  bolt 
for  clamping  them  to  the  holders. 

The  Bullock  Electric  Manufacturing  Company's  direct 
connected  generator,  Fig.  98,  has  an  external  appearance 
similar  to  that  of  the  generators  of  other  companies.  It 


is  different  from  them,  however,  in  having  peculiarly  con- 
structed poles.  These  poles  are  made  up  of  laminated 
steel  stampings,  which  are  much  thinner  than  are  ordi- 
narily used,  and  which  have  the  peculiar  shape  shown  in 
Fig.  99.  In  assembling  these  stampings  to  form  the  pole, 
every  alternate  one  is  reversed  from  the  position  which  is 


128 


DYNAMO   ELECTRIC   MACHINERY. 


indicated  in  the  figure.  The  method  of  assembling  is 
shown  in  Fig.  100.  After  assembling,  it  will  be  seen  that 
the  face  of  the  pole  for  a  short  depth  contains  but  one-half 
as  much  iron  as  the  main  body  of  the  pole.  This  results, 
under  normal  excitation,  in  a  saturated  pole  face.  It  has 
the  same  effect  in  preventing  distortion  of  the  field  under 
the  influence  of  armature  reaction,  as  saturation  of  the 
teeth  of  the  armature  core.  The  teeth  can,  therefore,  be 


Fig.  99- 


Fig.  100. 


operated  at  a  smaller  magnetic  flux  density.  The  hystere- 
sis losses  in  the  teeth  can  accordingly  be  made  smaller. 
The  thinness  of  the  stampings,  and  the  ideally  perfect 
lamination  of  the  pole  face,  permit  the  use  of  a  smaller 
ratio  of  tooth  width  to  slot  width,  without  the  excessive 
eddy  current  loss  in  the  pole  face  which  would  occur  in 
ordinary  machines.  The  possibility  of  using  narrow  teeth 
results  in  a  reduction  of  the  inductances  of  the  armature 
coils.  This  facilitates  effective  commutation. 


CONSTANT   CURRENT   DYNAMOS.  129 


-X X X X X- 


CHAPTER   VIII. 

CONSTANT  CURRENT   DYNAMOS. 

78.  Direct  Current  Arc  Lighting  Generators.  —  For 
lighting  by  arc  lights  where  considerable  energy  is  e:  - 
pended  at  the  points  of  illumination,  and  where  these 
points  are  separated  from  each  other  by  considerable  dis- 
tances, it  is  sometimes  economical  and  desirable  to  connect 
the  lamps  in  series  and  use  a  constant  current.  A  single 
line  then  completes  a  circuit  of  all  the  lamps  (Fig.  101). 
The  line  can  be  made 
of  much  smaller  wire 
than  in  the  case  of  a 
constant  pressure  cir- 
cuit, for  on  a  constant 
current  circuit  as  the 
load  increases  the  power 

or  energy  transmitted  is  Fig.  101. 

increased  by  raising  the 

potential,  the  current  remaining  unaltered  ;  while  in  a  con- 
stant pressure  circuit  an  increase  of  load  is  met  by  an 
increase  of  current,  and  the  wires  of  the  line  have  to  be  of 
sufficient  size  to  safely  carry  the  maximum.  The  size  of 
wire  necessary  is  dictated,  not  by  the  energy  transmitted, 
but  by  the  current  flowing,  hence  a  wire  large  enough  to 
supply  just  one  lamp  of  a  constant  pressure  circuit  can 
supply  all  the  lamps  of  a  constant  current  circuit. 


O 


130  DYNAMO  ELECTRIC  MACHINERY. 

The  more  general  forms  of  arc  lamps  have  what  is 
termed  a  spherical  candle-power  of  800,  1200,  or  2000. 
Lamps  used  in  search-lights  and  in  light-houses  often  ex- 
ceed this  in  candle-power,  and  may  consume  many  more 
amperes.  The  arc  lamp  of  800  candle-power  takes  a  cur- 
rent of  4.5  amperes,  that  of  1200  candle-power  6.6  to 
6.8  amperes,  and  that  of  2000  candle-power  9.6  to  9.8 
amperes. 

An  ordinary  arc  lamp,  as  it  is  trimmed  and  adjusted  for 
general  use,  requires  between  45  and  50  volts  to  force  its 
rated  current  through  it.  A  generator  supplying  a  circuit 
of  say  2000  candle-power  lamps  with  n  such  lamps  in  the 
circuit  must  be  capable  of  generating  a  constant  current  of 
9.8  amperes.  It  must  be  able  to  regulate  its  pressure 
between  the  limits  of  50  and  50/2  volts.  This  is  necessary 
in  order  that  it  may  operate  all  the  lamps  or  any  part  of 
the  whole  number. 

The  current  of  an  arc-light  machine  must  not  exceed 
nor  fall  below  its  normal  value,  no  matter  how  suddenly 
the  load  is  varied ;  for  the  slightest  change,  even  for  a  very 
brief  instant  of  time,  affects  the  quality  of  the  light  at  the 
lamps.  It  is  obvious  that  some  mechanical  device  could  be 
applied  to  an  ordinary  shunt -wound  generator  to  cause  it 
to  give  constant  current,  either  by  changing  the  position  of 
the  brushes  or  by  varying  the  ampere  turns  of  the  field 
coils.  However,  any  such  device  would  be  slow  of  opera- 
tion, and  a  sudden  short  circuit  would  cause  a  destructive 
current  to  flow  before  the  regulator  completed  its  action. 
It  is,  therefore,  necessary  to  rely  on  the  armature  reactions 
for  regulation,  since  they  vary  simultaneously  with  the  cur- 
rent. All  successful  constant  current  machines  are  con- 
structed on  this  principle.  The  machine  is  designed  with 


CONSTANT  CURRENT  DYNAMOS.       13! 

a  field  of  very  great  magnetizing  power,  the  armature  re- 
actions are  very  great,  and  thus  the  total  flux  effective  in 
producing  E.M.F.  is  reduced.  A  slight  increase  of  current 
in  the  armature  materially  increases  the  armature  reactions. 
The  effective  flux  is  thus  reduced,  and  the  pressure  falls 
until  the  current  returns  to  its  normal  value.  Thus  the 
machine  is  completely  and  instantly  self -regulating.  Since 
the  field  magnetization  is  kept  constant  and  the  machine 
produces  constant  current  the  field  coils  are  series  wound 
on  all  arc-light  generators,  and  the  cores  of  the  field 
magnets  are  worked  at  a  very  high  magnetic  density,  since 
the  magnets  are  then  more  insensible  to  slight  changes  in 
the  magnetizing  force.  In  commercial  machines  the  den- 
sities in  the  field  cores  are  from  17,000  to  18,000  lines  per 
square  centimeter  for  wrought  iron  or  steel,  and  from  9000 
to  1 1,000  lines  for  cast  iron. 

In  the  armature  high  magnetic  density  is  also  required 
to  prevent  a  sudden  rise  of  voltage  when  the  circuit  is 
broken.  With  no  current  in  the  armature,  the  total  mag- 
neto-motive force  of  the  field  magnets  would  be  effective 
in  producing  E.M.F.,  and  a  destructive  rise  of  pressure 
would  result,  since  the  total  M.M.F.  of  the  field  magnets 
is  much  greater  than  the  normal  effective  M.M.F.  But  a 
high  magnetic  density  in  the  armature  core  leaves  the  latter 
incapable  of  receiving  such  an  increase  of  flux,  and  there- 
fore destructive  voltages  are  avoided.  In  practice  the  arma- 
ture core  is  designed  to  have  a  density  of  from  1 5,000  to 
20,000  lines  per  square  centimeter  at  its  minimum  cross- 
section. 

A  consideration  of  the  foregoing  theory  of  regulation 
shows  that  the  following  conditions  should  obtain  more  or 
less  completely  in  a  successful  constant  current  generator : 


132  DYNAMO   ELECTRIC   MACHINERY. 

(a)  since  the  current  is  small,  there  must  be  a  great  num- 
ber of  armature  turns ;  (b)  the  magnetic  field  of  the  ma- 
chine must  be  much  distorted ;  (c)  the  path  of  the  lines  of 
force  of  the  field  coils  must  be  long  and  of  small  area,  so 
the  M.M.F.  cannot  be  readily  changed  ;  (d)  the  path  of  the 
lines  of  force  due  to  armature  magnetization  must  be  short 
and  of  great  area,  so  the  M.M.F.  of  the  armature  will  change 
with  the  slightest  change  of  current ;  and  (e)  the  pole  piece 
must  be  worked  at  a  high  density. 

Evidently  extreme  difficulty  is  found  in  so  designing  the 
different  parts  of  the  machine  as  to  give  proper  considera- 
tion to  each  of  the  conditions  and  yet  produce  a  machine 
that  will  regulate  for  constant  current  at  all  loads.  This 
leads  to  the  introduction  of  automatic  mechanical  devices 
for  aiding  in  the  regulation.  These  devices  must  not  be 
considered  as  being  the  sole  regulators,  for  in  every  case 
they  are  secondary  to  the  natural  self -regulating  tendency 
of  the  armature.  In  general  they  regulate  for  the  gradual 
and  greater  changes  of  load,  while  the  armature  reactions 
take  care  of  the  smaller  and  more  sudden  fluctuations. 

There  are  two  general  systems  of  regulating  arc  dynamos. 
The  first  method  is  to  cause  the  machine  to  develop  an 
E.M.F.  in  excess  of  that  required  for  the  load,  and  to  then 
collect  an  E.M.F.  just  sufficient  for  the  load  in  hand.  This 
is  done  by  shifting  the  brushes  from  the  neutral  plane  (§50). 
In  a  closed-coil  armature  this  causes  a  counter  pressure  to 
be  developed  in  those  conductors  lying  between  the  neutral 
and  the  commutating  planes.  This  reduces  the  pressure 
to  the  desired  amount.  In  an  open-coil  armature  the 
brushes,  when  in  the  maximum  position,  connect  to  the 
circuit  those  coils  of  the  armature  which  at  that  instant 
have  the  maximum  E.M.F.  generated  in  them.  By  shift- 


CONSTANT   CURRENT   DYNAMOS.  133 

ing  the  brushes  either  way  coils  can  be  connected  to 
the  circuit  which  have  some  E.M.F.  less  than  the  total 
E.M.F.  generated  in  them,  and  the  amount  of  shifting 
regulates  the  pressure  on  the  line. 

The  second  method  of  arc-light  dynamo  regulation  is  to 
vary  the  magnetizing  force  in  the  field  magnets  just  enough 
to  put  the  required  pressure  on  the  line.  Since  the  mag- 
netizing force  is  dependent  on  the  ampere  turns  of  the  field 
coils,  it  can  be  varied  either  by  cutting  out  or  short  circuit- 
ing some  of  the  turns  or  by  changing  the  current  in  them 
by  means  of  a  variable  resistance  which  is  shunted  across 
the  field  terminals.  In  practice  both  these  methods  have 
been  used. 

Whether  regulation  is  effected  by  changing  the  position 
of  the  brushes,  or  by  changing  the  field  excitation,  sparking 
will  occur  at  the  points  of  collection  of  the  current  if  means 
are  not  provided  to  avoid  it.  Non-sparking  collection  could 
be  obtained  if  the  field  were  perfectly  uniform  all  around 
the  armature.  In  general  this  condition  is  impracticable, 
since  it  requires  almost  the  whole  armature  to  be  covered 
by  the  pole  faces,  and  it  requires  the  density  in  the  gap 
beneath  them  to  be  uniform.  Considerations  of  magnetic 
leakage  and  armature  reaction  render  almost  impossible  the 
satisfying  of  these  conditions.  Another  and  more  prac- 
tical method  is  to  employ  for  collection  at  one  terminal  of 
the  machine  two  brushes  connected  in  parallel.  These  are 
moved  in  opposite  directions,  thus  giving  the  effect  of  a 
single  brush  of  varying  circumferential  contact,  whose 
center  can  always  be  kept  in  the  neutral  plane.  This 
prevents  bad  sparking.  The  device  is  used  quite  success- 
fully in  practice.  There  is,  however,  some  question  as  to 
the  advisability  of  resorting  to  it. 


134 


DYNAMO   ELECTRIC   MACHINERY. 


9.  The  Brush  Machine.  —  Fig.  102  shows  a  standard 
i6o-light  Brush  arc  generator,  made  by  the  General 
Electric  Company.  The  armature  revolves  between  the 


Fig.  102. 


opposed  pole  faces  of  two  sets  of  field  magnets.  Like 
poles  are  opposed  to  each  other.  The  flux,  therefore, 
takes  a  path  out  of  the  opposing  pole  faces  into  the  arma- 


CONSTANT  CURRENT  DYNAMOS. 


135 


ture  core,  and  then  circumferentially  through  the  core  and 
out  into  the  next  pair  of  opposing  pole  faces. 


Fig.  103. 


The  armature,  Fig.  103,  consists  of  a  number  of  coils  or 
bobbins  placed  on  a  ring  core  of  greater  radial  depth  than 
breadth,  and  the  pole  faces  cover  the  sides  instead  of  the 


Fig.  104. 


circumference.  The  bobbins  are  protected  by  an  insulat- 
ing box,  shown  in  Figs.  104  and  105,  but  are  not  surrounded 
by  any  masses  of  metal.  This  fact,  coupled  with  the  fact 
that  the  armature  is  of  .such  a  shape  as  to  cause  great  air 


136 


DYNAMO  ELECTRIC   MACHINERY. 


disturbance,  insures  exceptional  ventilation  of  the  armature, 
and  tends  to  prevent  the  "  roasting  out "  of  the  coils  when 
subject  to  an  overload.  This  machine  is  of  relatively  slow 
speed,  the  larger  sizes  running  at  only  500  R.P.M. 


Fig.  105. 


At  a  given  instant  of  time,  the  different  coils  on  the 
moving  armature  have  E.M.F.'s  of  widely  different  magni- 


Fig.  106. 


tudes  induced  in  them.  The  commutator,  Fig.  106,  is  so 
designed  that  it  connects  the  coils  of  highest  E.M.F.  in 
series  with  each  other  to  the  external  circuit,  and  con- 
nects the  coils  of  medium  E.M.F.  in  multiple  with  each 


CONSTANT  CURRENT  DYNAMOS. 


137 


other  to  the  external  circuit,  while  those  of  smallest  E.M.F. 
are  cut  out  entirely  from  the  circuit. 

The   bearings   are    self-lubricated  by    means   of   rings. 
Since  the    poles  are  on  the  sides  of  the  armature,  side 


Fig.  107. 


play  in  the  bearings  must  be  prevented.  To  this  end  the 
commutator  end  of  the  shaft  is  turned  with  six  thrust  col- 
lars, as  seen  in  Fig.  107,  which  are  engaged  by  correspond- 
ing annular  recesses  in  the  brasses. 


13* 


DYNAMO  ELECTRIC  MACHINERY. 


Regulation  on  these  machines  is  effected  by  a  variable 
resistance  in  shunt  with  the  field  coils;  and  as  the  field 
current  is  changed  the  position  of  the  brushes  is  also 
changed,  not  to  collect  current  at  a  lower  voltage  as  de- 
scribed in  §  78,  but  to  obtain  sparkless  collection.  These 
two  operations  are  performed  by  a  regulator  (Fig.  108), 


Fig.  108. 

which  is  attached  directly  to  the  frame  of  the  machine. 
The  mechanism  consists  of  a  rotary  oil-pump  driven  by  a 
belt  from  the  armature  shaft,  a  balance  valve  of  the  piston 
type,  and  a  rotary  piston  in  a  short  cylinder,  which  is 
directly  connected  to  an  arm  sweeping  the  contacts  of  the 
field-shunt  rheostat.  The  valve  is  operated  by  a  lever 


CONSTANT   CURRENT   DYNAMOS. 


139 


actuated  by  a  controlling  electro-magnet  which  is  energized 
by  the  whole  generator  current.  At  normal  current  the 
valve  is  centrally  placed,  and  the  oil  from  the  pump  flows 
around  the  overlapping  ports  into  the  reservoir  without 
effect  (See  Fig.  109).  If  the  current  rises  above  the  nor- 
mal, the  armature  of  the  controlling  magnet  is  attracted, 
the  balance  valve  moves  up,  and  oil  enters  the  cylinder, 


Fig.  109. 

moving  the  rotary  piston  in  a  clockwise  direction.  The 
shaft  of  this  piston  moves  the  arm  of  the  rheostat,  cutting 
out  resistance  and  thus  lowering  the  field  exciting  current. 
At  the  same  time  a  pinion  on  the  shaft,  seen  in  Fig.  1 10, 
actuates  a  rocker  arm  which  moves  the  brush  holders  to  a 
position  such  that  the  collection  by  the  brushes  will  be 
sparkless.  When  the  current  returns  to  normal  the 
adjusting  spring,  seen  in  Fig.  1 1 1,  returns  the  lever  and 
balance  valve  to  the  central  position.  If  the  current  falls 


140  DYNAMO  ELECTRIC  MACHINERY. 


Fig.  in. 


CONSTANT   CURRENT   DYNAMOS.  141 

below  the  normal,  these  operations  are  reversed.  The 
tension  of  the  adjusting  spring  can  be  regulated  from  the 
outside  of  the  dust-proof  case  by  a  hard  rubber  knob. 
From  the  nature  of  the  case  the  parts  are  always  well 
lubricated. 

It  is  claimed  for  this  regulator  that  it  can  bring  the  cur- 
rent back  to  normal  from  a  dead  short-circuit  in  from  3  J  to 
4  seconds. 

80.   The  Westinghouse  Arc-Light   Machine Fig.   1 1 2 

shows  a  75 -light  direct  current  arc-lighting  generator, 
made  by  the  Westinghouse  Electric  and  Manufacturing 
Company.  It  is  of  rigid  construction,  the  bearing  sup- 
ports being  cast  integral  with  the  frame.  For  facilitating 
transportation  and  repairs  the  yoke  parts  in  the  middle 
on  a  horizontal  plane.  The  bearings  are  of  the  self- 
oiling,  self-aligning  type  described  in  §  41.  The  armature 
shown  in  Fig.  1 1 3  is  of  the  open-coil  type,  which  gives  a 
unidirectional  but  not  absolutely  continuous  current.  The 
slight  pulsations  of  the  current  thus  set  up,  while  not 
affecting  the  steady  mean  value  of  the  current,  cause  a 
slight  constant  vibration  in  the  mechanism  of  the  lamps 
that  helps  overcome  any  tendency  to  stick  or  a  failure  to 
feed  the  carbons.  A  unique  feature  of  this  armature  con- 
sists in  its  having  two  separate  sets  of  windings  on  the 
same  core,  each  set  having  its  own  commutator.  The  coils 
and  commutators  are  so  arranged  that  while  a  set  of  coils 
of  one  winding  is  being  cut  into  or  out  of  the  curcuit  a  set 
of  the  other  winding  is  supplying  current  to  the  line.  It  is 
claimed  for  this  method  of  connecting  open-coil  armatures, 
that  it  yields  a  more  satisfactory  current,  and  obviates  the 
vicious  sparking  at  the  commutator  found  in  other  types 


142 


DYNAMO  ELECTRIC  MACHINERY. 


of  open-coil  machines.  The  armature  is  made  of  lami- 
nated steel  sheets  punched  with  T-shaped  teeth  between 
the  winding  slots.  The  armature  coils  are  wound  on 


; 


Fig.  112. 

molds,  and  insulated  and  mounted  on  the  armature  as 
shown  in  Fig.  1 1 4.  They  are  held  in  place  by  wooden 
wedges  forced  into  the  loops  left  at  the  ends  of  the  arma- 
ture. This  construction  admits  of  removing  one  coil  for 
repairs  without  disturbing  any  of  the  other  coils. 


CONSTANT  CURRENT  DYNAMOS. 


143 


This  machine  differs  from 
the  general  type  of  arc-light- 
ing machines  in  that  it  is 
separately  excited,  a  small 
auxiliary  machine  generating 
current  for  the  field  coils  at 
100  volts.  This  obviates  the 
possibility  of  danger  from  a 
too  high  pressure  resulting 
from  an  open  circuit. 

Regulation  is  obtained  by 
careful  design,  so  that  the 
armature  reactions  cause  the 
voltage  to  vary  in  just  the 
proper  proportions,  as  de- 
scribed in  §  78.  The  exciting 
field  current  is  regulated  to 
give  the  proper  excitation  by 
a  series  rheostat.  By  this 
means  the  line  current  can 
be  raised  or  lowered  slightly 
if  desired,  without  affecting 
the  self-regulation. 

Fig.  1 1 5  shows  the  double 
commutator  of  this  machine. 
The  segments  are  easily  re- 
moved and  replaced  in  case 
they  wear  or  burn  out. 


81.  The  Wood  Arc  Dynamo.  —  Fig.  116  shows  a  Wood 
constant  current  dynamo  for  lighting  125  2000  c.p.  lamps. 
This  machine  claims  an  efficiency  of  90  per  cent  on  full 


144 


DYNAMO   ELECTRIC   MACHINERY. 


load.  The  bearings  are  self -oiling,  and  may  be  removed 
for  repairs  or  inspection  without  removing  the  armature. 
The  armature  has  large  radiating  surface  and  shallow  wind- 


Fig.  n4. 


ing,  and  its  temperature  does  not  rise  more  than  40°  C. 
above  the  temperature  of  the  room.  This  armature  is  of 
the  closed-coil  type,  requiring  a  commutator  of  many  seg- 
ments with  but  a  small  potential  between  any  two  adjacent 


CONSTANT   CURRENT   DYNAMOS. 


145 


ones.     This  fact,  and  the  use  of  two  brushes  in  parallel,  as 
explained  in  §  78,  obviates  all  sparking. 

This  machine  operates  by  generating  full  pressure  at  all 
times,  and  by  automatically  setting  the  brushes  to  take  off 
just  such  potential  as  is  necessary.  This  allows  of  "regu- 
lation without  making  use  of  rheostats,  separate  ex- 
citers, wall  controllers,  motors,  or  relays.  The  regulating 


Fig.  115. 


mechanism  is  set  in  operation  by  a  sensitive  and  rather 
powerful  electro-magnet  excited  by  the  main  armature 
current.  This  attracts  a  lever  which  is  restrained  by  an 
adjustable  coiled  spring.  A  variation  in  the  current 
strength  causes  this  lever  to  throw  into  train  one  or  the 
other  of  two  oppositely  revolving  fiber  friction  cones,  which, 
acting  through  gears  and  levers,  shifts  the  brushes  the  re- 


146  DYNAMO   ELECTRIC  MACHINERY. 

quisite  amount,  and  also  varies  their  angular  contact  or 
collecting  extent.  All  the  delicate  parts  of  this  mechanism 
are  inclosed  in  the  pillar  supporting  the  commutator  end 
of  the  armature  shaft,  and  are  thereby  protected  from 


Fig.  116. 

injury,  dust,  and  grit.  The  wearing  surfaces  of  this  regu- 
lator are  all  large  and  the  speed  is  slow,  so  that  wear  is  re- 
duced to  a  minimum.  Without  any  change  of  adjustments 
this  regulator  will  operate  when  run  either  way,  which  is 
an  advantage  when  two  or  more  dynamos  are  run  from  one 
engine,  and  economy  of  space  is  essential,  or  in  case  of 
accident  to  a  prime  mover. 


CONSTANT   CURRENT    DYNAMOS. 


147 


82.  The  Excelsior  Arc  Dynamo. — This  machine,  Fig. 
1 1 7,  is  a  closed-coil  ring  armature  generator,  having  pole 
faces  that  cover  both  the  sides  and  the  circumference  of 
the  armature.  The  interesting  feature  of  this  machine  is 
the  method  of  regulation.  The  proper  potential  is  sup- 
plied to  the  line  by  using  both  methods  of  control  in  con- 
junction ;  that  is,  sections  of  the  field  windings  are  cut  in 
or  out  of  circuit,  and  at 
the  same  time  the  posi- 
tion of  the  brushes  is 
shifted.  The  proper 
motion  of  the  field  regu- 
lator arm  and  of  the 
brush  holder  is  obtained 
by  means  of  a  small 
motor  whose  field  is 
"  sneaked "  from  the 
main  magnets  of  the 
machine.  This  motor 
is  operated  by  a  device 
shown  in  Fig.  118.  The 
whole  device  is  inserted 
in  series  with  one  of  the 
mains  from  the  gener- 
ator. The  right-hand 
lever  is  of  insulating  material,  with  the  contact  blocks 
a  and  b  properly  placed  upon  it.  The  left-hand  lever 
is  of  conducting  material,  and  is  capable  of  being  attracted 
by  the  electro-magnet  which  is  excited  by  the  main 
current.  The  magnet  and  spring  are  so  adjusted  that 
when  the  normal  current  is  flowing,  both  a  and  b  are 
in  contact  with  the  left  lever,  and  the  current  flows  in  the 


Fig.  117. 


DYNAMO  ELECTRIC  MACHINERY. 


three  shunt  paths,  Ry  Rv  and  Rz.  There  will  be  no  cur- 
rent in  the  armature  of  the  regulating  motor,  since  the 
potential  at  brush  x  is  equal  to  the  potential  at  brush  y. 
If  now  the  line  current  becomes  too  strong  the  magnet 
attracts  the  left  lever  to  it  and  the  contact  at  a  is  broken. 


KV  >/l 


To  Line 


From  Dynamo 


Fig.    Il8. 


Immediately  the  current  flowing  through  b  divides  at  the 
brush  x,  part  going  through  J\z  and  part  through  the  motor 
armature  and  F^.  The  motor  will  then  revolve  in  a  given 
direction,  and  by  simple  mechanical  devices  will  cut  out 
sections  of  the  field  windings,  and  will  shift  the  brushes 
until  the  normal  current  is  flowing,  when  contact  is  again 


CONSTANT   CURRENT   DYNAMOS.  149 

made  at  a  and  the  controlling  motor  stops.  If  the  line 
current  drops  below  normal,  the  spring  pulls  the  lever 
away  from  the  magnet  and  the  contact  at  b  is  broken. 
Part  of  the  current  then  flows  from  y  to  x  through  the 
motor  armature.  It  therefore  revolves  in  a  direction  op- 
posite to  that  which  it  had  before.  The  brushes  on  the 
dynamo  are  shifted  back  again,  and  more  sections  of  field 
winding  are  put  into  circuit. 

In  practice  the  levers  and  the  magnet  are  mounted  on 
the  wall  or  the  switch-board,  while  the  regulating  motor  is 
mounted  on  the  dynamo  frame. 

When  the  current  is  broken  at  a  or  b,  there  is  no  serious 
sparking,  since  there  are  always  two  circuits  in  shunt  with 
the  break.  The  whole  current  of  the  dynamo  does  not 
exceed  ten  amperes  ;  and  the  resistances  R,  Rv  and  Rz  are 
so  proportioned  that  only  a  small  portion  of  that  flows 
through  a  or  b. 

83.  The  Ball  Arc  Generator. —  Fig.  119  shows  a  double 
armature,  automatic  regulating  constant  current  generator, 
made  by  the  Ball  Electric  Company.  Two  independent 
circuits,  each  automatically  controlled,  can  be  operated 
from  the  one  machine,  since  it  has  two  distinct  armatures, 
commutators,  arid  regulators.  The  advantage  claimed  for 
this  arrangement  is  that  the  pressure  has  to  be  but  half  as 
high  as  if  the  two  circuits  were  united  and  fed  by  a  single 
armature.  Yet  if  it  be  undesirable  to  bring  the  ends  of 
two  circuits  into  the  power-house,  they  can  be  connected 
in  series,  and  fed  by  the  two  armatures  also  connected  in 
series,  and  then  the  voltage  per  armature  will  be  half  that 
of  a  single  armature  machine  giving  like  results. 

The  armatures  are  of  the  closed-coil  ring  type.     The  air 


150  DYNAMO   ELECTRIC   MACHINERY. 


CONSTANT   CURRENT   DYNAMOS.  151 

gap  between  pole  faces  and  armature  is  short  in  length  and 
great  in  area,  requiring  a  minimum  of  magnetic  excitation. 
The  commutator  is  built  up  of  a  great  number  of  segments, 
the  potential  between  any  two  adjacent  segments  not  ex- 
ceeding fifteen  volts.  This  assures  sparkless  commutation. 

This  generator  is  regulated  by  shifting  the  brushes  until 
pressure  of  a  suitable  magnitude  is  collected. 

A  magnetic  body  placed  in  a  magnetic  field  will  tend  to 
rotate  until  the  longest  axis  is  parallel  to  the  magnetic  lines 
of  force.  This  principle  is  applied  to  the  Ball  regulator  as 
follows  :  A  magnetic  portion  of  the  brush  carrier  is  made  a 
part  of  the  magnetic  circuit,  and  is  placed  in  a  recess  of  the 
dynamo  frame.  It  tends  to  assume  an  axial  position  with 
a  force  varying  as  the  flux  through  it.  As  the  line  current 
increases  the  flux  increases,  and  the  brush  holder,  which  is 
mounted  on  ball  bearings,  rotates,  shifting  the  brushes  the 
required  amount.  The  impulse  to  regulate  is  applied 
directly  to  the  brush  holder,  instead  of  being  communi- 
cated to  it  by  by  a  more  or  less  complex  mechanism.  The 
magnetic  tendency  to  shift  the  brush  holder  is  opposed  by 
gravity. 

84.  The  Thomson-Houston  Dynamo.  —  The  Thomson- 
Houston  arc  generator  is  of  a  type  entirely  different  from 
the  other  machines  here  described,  not  only  in  appearance, 
but  also  in  method  of  armature  winding  and  of  regulation. 
A  view  of  this  machine  is  given  in  Fig.  1 20.  Each  field 
coil  has  for  its  core  an  iron  tube,  flanged  exteriorly  at  each 
end  to  form  a  recess  for  the  windings,  and  fitted  at  the 
armature  end  with  a  concave  iron  piece  that  surrounds  part 
of  the  armature.  This  tube,  with  the  flanges  and  the  cup- 
shaped  end.,  is  cast  in  one  piece.  The  farthermost  flange 


152  DYNAMO   ELECTRIC    MACHINERY. 

of  each  field  core  is  bolted  to  a  number  of  wrought-iron 
connecting-rods  which  hold  the  magnets  in  place,  protect 
the  field  windings,  and  take  the  place  of  the  yoke  of  other 
machines  in  completing  the  magnetic  circuit.  The  mag- 
nets are  mounted  on  a  frame,  including  legs  and  bearing 
supports  for  the  armature  shaft. 

The  armatures  of  the  older  machines  of  this  type  are 
spheroidal  in  shape,  while  the  more  recent  ones  have  ring 
armatures  which  are  more  readily  repaired  or  rewound. 
The  winding  of  either  form  of  armature  is  peculiar  in  that 
only  three  coils  are  employed,  set  with  an  angular  dis- 
placement from  one  another  of  120  degrees.  In  the  ring 
armature  no  difficulty  is  found  in  properly  winding  these 
coils ;  but  in  the  old  spherical  armature  the  following  de- 
vice was  employed  to  secure  the  windings,  and  give  each 
of  them  the  same  average  distance  from  the  pole  face.  A 
hollow  spheroidal  iron  core  was  keyed  on  the  shaft.  The 
core  had  three  rows  of  externally  projecting  wooden  pins. 
Between  these  pins  the  coils  were  wound,  half  of  coil  A 
being  wound  first,  then  at  1 20  degrees  distance  half  of  coil 
B  was  wound,  covering  parts  of  coil  A.  Then  at  120  de- 
grees from  both  A  and  B  all  of  coil  C  was  wound.  Over 
this,  but  in  its  proper  angular  position,  the  other  half  of 
coil  B  was  wound,  and  finally  the  rest  of  coil  A  was  put  in 
place.  By  this  arrangement  the  average  distance  of  each 
coil  from  the  pole  face  or  from  the  iron  core  was  the  same. 
In  either  type  of  armature  the  inner  ends  of  the  three  coils 
are  joined  to  each  other,  and  are  not  attached  to  any  other 
conductor,  an  arrangement  unique  in  direct  current  dyna- 
mos. The  outside  ends  are  connected  to  the  segments  of 
a  three-bar  commutator,  from  which  the  current  is  collected 
by  four  copper  brushes  connected  in  multiple. 


CONSTANT  CURRENT  DYNAMOS. 


153 


Regulation  is  obtained  by  shifting  the  brushes  in  the 
following  manner.      Fig.  1 2 1  shows  the  two  possible  rela- 


Fig.  120. 


tions  between  brushes  and  commutator  that  may  exist  at 
any  instant.  Both  brushes  of  each  set  may  rest  on  one 
commutator  bar,  or  the  brushes  of  one  set  may  span  the 


154 


DYNAMO   ELECTRIC   MACHINERY. 


break  between  two  of  the  bars.  These  conditions  are  re- 
peated three  times  at  each  brush  for  each  revolution.  If 
the  dotted  line  shows  the  position  where  the  maximum 
E.M.F.  is  generated  in  the  coils,  then  in  Fig.  121  a  the 
two  most  active  coils  are  connected  in  series  with  the  out- 
side circuit,  while  the  coil  near  the  position  of  least  activity 
is  out  of  circuit.  In  Fig.  i2ib  the  two  less  active  coils  are 
in  multiple  with  themselves  and  in  series  with  the  most 
active  coil  and  the  external  circuit.  In  practice  the 


Fig.  121. 

brushes  of  a  set  are  60  degrees  apart,  leaving  120  degrees 
between  the  leading  brush  of  one  set  and  the  following 
brush  of  the  other  set ;  and  since  1 20  degrees  is  the  angular 
measure  of  the  length  of  a  commutator  bar,  there  is  no 
coil  out  of  circuit  at  normal  load,  two  being  always  in 
parallel  and  in  series  with  a  third.  If  the  current  rise 
above  the  normal  the  leading  brushes  move  a  small  angle 
forward,  while  the  following  brushes  recede  through  three 
times  that  angle.  This  will  shorten  the  time  that  a  single 
coil  gives  its  whole  E.M.F.  to  the  circuit,  and  will  place  it 
more  quickly  in  parallel  with  a  comparatively  inactive  coil. 
But  such  a  movement  will  reduce  the  angular  distance  be. 


CONSTANT  CURRENT   DYNAMOS. 


155 


tween  tne  nearest  brushes  of  the  opposite  sets  to  less  than 
1 20  degrees,  hence  the  machine  will  be  short  circuited  six 
times  per  revolution,  since  one  brush  of  each  set  will  touch 
one  segment  of  the  commutator  at  the  same  time.  If  the 
current  in  the  line  falls  below  normal,  then  the  brushes 
close  together,  and  the  time  that  a  coil  is  in  series  is 
lengthened,  and  the  time  that  it  is  in  parallel  with  an 
inactive  one  is  lessened. 


field 


The  arrangement  for  moving  the  brushes  is  shown  in 
Fig.  122.  The  leading  brushes  are  shifted  forward  on  an 
increase  of  current  merely  to  help  avoid  sparking.  The 
brushes  are  moved  by  levers  actuated  by  a  series  magnet 
A.  This  magnet  is  normally  short  circuited  by  the  by- 
pass circuit.  On  an  undue  rise  of  current  this  circuit  is 
broken  by  the  series  magnet  B.  A  then  becomes  more 
powerful,  and  the  levers  separate  the  brushes.  While  the 
machine  is  in  operation  the  circuit-breaker  C  is  constantly 
vibrating,  and  brushes  adjusting  to  suit  the  load.  A  high 


156  DYNAMO   ELECTRIC   MACHINERY. 

carbon  resistance  is  shunted  across  C  to  prevent  sparking 
at  that  point. 

As  might  be  expected,  with  but  three  parts  to  the  com- 
mutator and  collection  made  with  small  regard  to  the 
neutral  point,  the  sparking  of  this  machine  is  such  as  to 
speedily  ruin  the  commutator  and  the  brushes,  if  means 
are  not  taken  to  suppress  it.  A  rotary  blower  is  mounted 
on  the  shaft,  and  is  arranged  to  give  intermittent  puffs  of 
air,  which  at  the  right  moment  blow  out  the  spark.  The 
insulation  between  the  segments  is  air,  considerable  gap 
being  left  between  them,  and  through  these  gaps  the 
sparks  are  blown. 

85.  Western  Electric  Arc  Dynamo.  —  Fig.  123  represents 
this  machine,  which  is  regulated  by  means  of  shifting  the 
brushes.  The  brush  and  rocker  are  connected  by  means 
of  a  link  and  a  ball  and  socket  joint  with  a  long  screw. 
This  screw  is  held  in  position  by  a  nut.  When  the  current 
is  normal,  both  the  nut  and  screw  revolve  at  the  same 
rate,  and  consequently  there  is  no  end  movement  of  the 
screw.  The  brush,  therefore,  remains  stationary.  An 
electro-magnet,  energized  by  a  coil  which  is  in  series  with 
the  main  circuit,  attracts  an  armature  whose  movement 
towards  the  magnet  is  opposed  by  the  action  of  a  spring 
which  is  susceptible  of  regulation.  When  the  current  has 
too  high  a  value,  the  electro-magnet  attracts  its  armature 
more  strongly  than  ordinarily.  The  latter  moves  toward 
the  magnet,  and  by  its  movement  catches  a  stop  on  the  re- 
volving nut,  and  thereby  prevents  the  revolution  of  the  nut 
until  the  resulting  longitudinal  movement  of  the  screw  has 
shifted  the  brushes  sufficiently  to  bring  the  current  to  its 
normal  value.  If  the  current  be  too  weak,  the  spring 
which  is  attached  to  the  magnet  armature  overpowers  the 


CONSTANT   CURRENT   DYNAMOS.  157 

electro-magnetic  attraction.  The  resulting  movement  of 
the  armature  stops  the  rotation  of  the  screw  and  permits 
the  rotation  of  the  nut.  This  results  in  a  longitudinal 
movement  of  the  screw  and  a  shifting  of  the  brushes  in 
the  opposite  direction.  The  stopping  and  starting  of  the 
nut  and  screw  is  accomplished  through  the  medium  of 


Fig.  123. 


small  triggers  controlled  by  the  armature  of  the  series 
magnet.  The  triggers  are  fastened  to  a  gear  rotated  from 
the  main  shaft  by  a  belt.  They  engage  with  stops  on  the 
nut  and  screw  respectively. 

Fig.  124  gives  a  sectional  view  of  the  regulator,  and  the 


158  DYNAMO   ELECTRIC   MACHINERY. 


Fig.    124. 


CONSTANT   CURRENT   DYNANOS.  159 


160  DYNAMO   ELECTRIC   MACHINERY. 

trigger  which  engages  with  the  screw  is  shown  at  n,  and 
the  one  which  engages  with  the  nut  is  shown  at  m. 

Fig.  125  represents  the  details  of  the  armature  con- 
struction. The  latter  is  ring  wound  with  a  large  number 
of  turns  in  each  section. 


MOTORS.  161 


CHAPTER    IX. 

MOTORS. 

86.  Principle  of  the  Motor Any   direct    current  dy- 
namo will  act  as  a  motor  if  supplied  with  current  from  some 
external  source.     This  source  may  be  a  constant  E.M.F. 
system,  a  constant   current  system,  or  any  other  system. 
The  rotation  of  the  armature  is  a  direct  consequence  of  the 
conditions   laid  down  in   §  20.       It  is  evident  that  if  the 
negative  and  positive  terminals  of  a  dynamo  be  connected 
with  the  corresponding  terminals  of  some  external  source 
of  supply,  the  direction    of  flow  in  the  armature  will  be 
reversed.     Irrespective  of  the  multipolarity  of  the  field  or 
of  the  method  of   armature  winding,  the    electro-dynamic 
actions  between  the  field  and  all  the  currents  in  the  in- 
ductors will  conspire  to  produce  rotation  in  one  direction. 

87.  Direction  of  Rotation To  determine  the  direction 

of  movement  of  an  inductor  carrying  a  current  of  known 
direction  in  a  magnetic  field  of  known  direction,  one  may 
employ  a  modification  of  Fleming's  rule.    Thus  in  a  dynamo 
the  thumb  and  two  first  fingers  of  the  right  hand  deter- 
mine the  direction  of  induced  E.M.F.  as  shown  in  Fig. 
1 26.     But  in  a  motor  the  thumb  and  two  first  fingers  of  the 
left  hand  can  be  made  to  determine  the  direction  of  motion 
as  shown  in  Fig.  127. 

If  in  a  dynamo  the  direction  of  the  field  flux  be  not 


162 


DYNAMO   ELECTRIC   MACHINERY. 


altered,  and  if  the  armature  be  supplied  with  a  current  flow- 
ing in  the  same  direction  as  when  the  machine  was  operated 
as  a  dynamo,  the  direction  of  rotation  will  be  reversed. 
Thus,  if  the  positive  brush  of  a  dynamo  be  connected  to 
the  positive  terminal  of  an  external  source  of  supply,  and  if 
the  negative  brush  be  connected  to  the  negative  terminal, 
then  the  direction  of  current  flow  in  the  armature  will  be 
reversed.  The  direction  of  rotation  of  the  armature,  in 


FICLO    MAGNET 


FIELD   MAGNET 

DYNAMoTlRIGHT  HAND. 


Fig.  126. 


MOTOR   LEFT   HAND.1 

Fig.  127. 


series-wound  machines,  since  the  field  flux  has  its  direction 
also  changed,  will  be  reversed.  In  shunt-wound,  separately 
excited,  and  magneto  machines,  since  these  do  not  have 
their  fields  reversed,  the  direction  of  rotation  will  be  un- 
altered. Compound-wound  machines  will  have  the  same 
or  reversed  direction  of  rotation,  depending  upon  whether 
the  magnetizing  effect  of  the  shunt  coils  is  stronger  or 
weaker  than  that  of  the  series  coils.  In  a  compound  gen- 
erator the  actions  of  the  shunt  coils  and  the  series  coils  are 
cumulative,  i.  e.,  in  the  same  direction  ;  but  when  used  as  a 
motor  the  actions  are  differential,  i.  e.,  opposed  to  each  other. 
Motors  are  also  wound  so  as  to  have  cumulatively  acting 
series  coils. 


MOTORS.  163 

88.  Speed  Conditions If  an  electric  motor  be  supplied 

with  electrical  energy,  it  will  vary  its  rate  of  rotation  until 
it  has  attained  such  a  speed  as  will  produce  an  equality  be- 
tween the  input  of  energy  and  the  output  of  energy.     The 
latter  appears  both  as  useful  work  and  as  losses.     In  the 
case  of  a  motor,  speed  acts  toward  electrical  energy  like 
temperature   in    the  case  of   heat    energy.     Temperature 
always  rises  until  the  heat  energy  which  is  produced  is 
equal  to  the  heat   energy  which  is  disposed    of    by  con- 
duction, convection,  and  radiation. 

The  electrical  energy  which  is  communicated  to  a  motor 
is  transformed,  a,  into  useful  mechanical  energy,  which  is 
taken  from  the  armature  shaft  either  by  a  belt  or  by  direct 
connection ;  b,  into  friction  at  the  bearings  and  at  the 
brushes;  r,  into  windage;  d,  into  foucault  and  eddy  currents; 
and  finally  e,  into  ohmic  heat  energy  in  the  motor's  electrical 
circuits.  The  energy  required  per  unit  of  time  to  overcome 
friction,  windage,  hysteresis,  and  foucault  and  eddy  currents 
increases  as  the  speed  of  rotation  increases.  Nearly  all 
practical  loads  put  upon  a  motor  —  machinery  in  one  form 
or  another  —  require  an  increase  of  power  for  an  increase 
of  speed.  Therefore,  if  a  given  amount  of  electrical  power 
be  communicated  to  a  motor  under  load,  the  armature  will 
assume  some  speed  of  rotation,  so  that  a  balance  between 
the  input  and  the  output  of  energy  is  maintained. 

89.  Counter  E.M.F.  —  If  the  variation  of  losses  and  useful 
energy  with  the  speed  were  the  only  conditions  governing 
the  speed,  then  there  would  result  in  practice  variations  of 
speed  through  enormous  ranges.     But  there  is  another  con- 
dition affecting  the  speed.     The  armature,  by  varying  its 
speed,  not  only  governs  the  rate  of  expenditure  of  energy, 
but  also  governs  the  amount  of  electrical  energy  received. 


164  DYNAMO   ELECTRIC   MACHINERY. 

The  armature  of  a  motor  revolving  in  a  field  under  the 
influence  of  supplied  electrical  energy  differs  in  no  respect 
from  the  same  armature  revolving  in  a  field  under  the  in- 
fluence of  supplied  mechanical  energy.  There  is  an  E.M.F. 
generated  in  it  which  is  determined  by  the  speed  and  quan- 
tity of  flux.  For  the  same  speed  and  the  same  flux  there 
would  be  generated  the  same  E.M.F.  in  the  case  of  a  motor 
as  in  the  case  of  a  dynamo.  The  direction  of  this  E.M.F. 
is,  however,  such  as  to  tend  to  send  a  current  in  a  direction 
opposite  to  that  of  the  current  flowing  under  the  influence 
of  the  external  supply  of  E.M.F.,  according  to  §  87. 
Therefore  this  pressure  which  is  induced  in  the  armature  of 
a  motor  is  called  counter  electro-motive  force.  The  current 
which  will  flow  through  the  inductors  of  an  armature  is 
therefore  equal  to  the  difference  between  the  supply  E.M.F. 
and  the  counter  E.M.F.  divided  by  the  resistance  of  the 

armature,  or  _        _ 

T        E*  —  Ec 

a    ~^r 

For  example,  an  unloaded  i  K.W.  shunt  motor  having  an 
armature  resistance  of  I  ohm,  when  connected  to  a  con- 
stant source  of  potential  supply  of  100  volts,  would  not  take 
a  current  of  100  amperes  as  dictated  by  Ohm's  law,  unless 
its  armature  were  clamped  so  as  to  prevent  rotation.  If 
undamped,  its  armature  would  assume  such  a  speed  that  it 
would  have  induced  in  it  a  counter  E.M.F.  of  say  97.5 
volts.  The  current  then  flowing  in  the  armature  would  be 

ioo  —  07. c 

-  =  2.5  amperes. 

The  power  represented  by  this  current,  viz.,  2.5  X  ioo 
watts,  would  all  be  expended  in  overcoming  the  losses  of 
the  machine. 


MOTORS. 


I6S 


90.  Armature  Reactions Since  in  a  motor,  for  a  given 

direction  of  rotation  and  flux,  the  current  in  the  armature 
flows  in  a  direction  contrary  to  that 

which  it  would  have  as  a  dynamo, 
therefore  the  effect  of  the  motor 
armature  cross  turns  is  to  skew  the 
field  against  the  direction  of  rotation, 
as  in  Fig.  128.  Tnis  increases  the 
magnetic  density  in  the  leading  pole 
tip,  and  decreases  it  in  the  trailing 
tip.  This  necessitates,  for  sparkless 
operation,  a  backward  lead,  or  a  lag, 
of  the  brushes.  If  the  brushes  were 
in  the  same  place  as  when  the  ma- 
chine was  operated  as  a  generator, 
the  direction  of  armature  current 
having  been  reversed,  then  the  de- 
magnetizing or  back  turns  of  the 
generator  would  become  magnetiz- 
ing turns  for  the  motor ;  but  with  the  brushes  shifted  to 
a  position  of  lag,  then  the  motor  has  also  demagnetizing 
or  back  turns. 

9 1 .  Efficiency —  In  a  compound- wound  motor  connected 
to  a  constant  pressure  supply, 

Let  R&  —  resistance  of  shunt  field  coils, 

^a+8  =  resistance  of  armature  plus  that  of  the  series  field 

coils, 

V=  number  of  revolutions  per  minute, 
T  =  torque  given  off  at  the  pulley  in  pound  feet, 
E  —  supply  voltage, 

fn  =  no-load  armature  current  in  amperes,  and 
/  =  armature  current  when  torque  T  is  yielded. 


Fig.  128. 


166  DYNAMO   ELECTRIC   MACHINERY. 

useful  power  output 

The  efficiency  =  —  -  :  —  J—          —  =-  --  , 
electrical   power  input 

hence, 


The  useful  power  can  be  expressed  as  the  difference 
between  the  power  input  and  the  losses.  Now  at  no  load, 
when  there  is  no  useful  power  output,  the  following  rela- 

tions exist  :  ^ 

EIn  -f  —  -  =  power  input, 
<Ksh 

and 

I^Ra+s  =  Pr  =  power  in  the  armature  and  series  coils. 

Assuming  the  friction,  the  windage,  the  foucault  current, 
and  the  hysteresis  losses  to  be  constant  and  the  same  as 
at  no  load,  we  have  for  their  value  a  constant  loss  =  the 
no-load  power  input  —  the  variable  loss. 
Hence, 

The  constant  loss  =  Pf  =  EIn  +  Psh  -  P'  , 

where  ^ 

Psh  =  loss  in  shunt  coils  =  —  -  • 

^sh 

The  efficiency  under  load  will  therefore  be 


El  +  PA 

In  a  shunt  motor  Ra  +  s  represents  the  armature  resistance 

only ;  hence, 

I 
e  =  - 


In  a  series  motor  Psh  =  o  ;  hence, 


MOTORS. 


I67 


In  the  first  of  these  three  expressions  for  efficiency, 
solving  for  that  value  of  /  which  will  give  a  maximum  effi- 
ciency, we  have 

<&         (EI+Psh}  (E  -  2  S£a+s)  -  (EI-Pf-I*£a+8)E  _ 


dl 

whence 


EI+Psh 


=  o, 


, 

"  E*  "  E' 


Fig.  129  gives  a  set  of  curves  indicating  the  perform- 
ance  of  a  motor  whose  fixed  losses  are  large.     Fig.  130 


Fixed/Losses  in  Shunt  Coils,  friction,  foucault,  and  hysteresis. 


Pouter   Input 

Fig.  129. 

gives  a  set  of  curves  for  an  exactly  similar  machine  save 
that  the  fixed  losses  are  smaller.  They  might  be  con- 
sidered as  taken  from  the  same  machine  as  the  first,  but 
with  journals  better  oiled,  and  hence  with  less  friction  loss. 
The  difference  in  the  efficiency  curves  is  noticeable. 


i68 


DYNAMO   ELECTRIC   MACHINERY. 


Abscissae  in  all  cases  represent  power  input.  The  ordi- 
nate  of  P  at  any  given  load  shows  the  power  input  at  that 
load.  The  constant  ordinate  of  F  represents  the  power 
consumed  by  the  fixed  losses,  which  is  constant  for  all 
loads.  The  ordinates  of  V,  measured  from  F,  follow  the 


Fixed  Losses  in  Friction,  foucau It,  hysteresis  and  shunt  coils* 


Pouter   Input 

Fig.  130. 

law  PRa+sy  and  represent  the  variable  loss  at  various  loads. 
Therefore  the  total  loss  for  any  load  is  represented  by  the 
total  ordinate  of  Fat  that  load.  The  difference  between 
the  power  input  and  the  losses  gives  the  useful  power, 
which  is  represented  by  the  difference  of  the  ordinates  of 

P  and  V.     The  values  of  the  ratio  power  ^   ^^ 

power  input 

load  are  plotted  in  the  efficiency  curve.  Comparing  the 
curves  of  the  two  machines,  it  is  seen  that  to  get  a  high 
efficiency  at  full  load  the  variable  loss  must  be  kept  small, 
while  to  obtain  a  good  efficiency  at  small  loads,  the  fixed 


MOTORS. 


169 


losses  must  be  made  small.  The  shape  of  the  efficiency 
curve  can  be  controlled  by  a  proper  adjustment  of  the 
relation  which  exists  between  the  fixed  and  the  variable 
losses. 

92.    Starting  Rheostats When  the  armature  of  a  motor 

is  at  rest  there  is  no  counter  E.M.F.;  and  at  the  instant  of 
closing  the  circuit  a  destructive  current  would  flow  if  a  re- 
sistance were  not  first  inserted  in  the  circuit,  except  in  the 
case  of  very  small  motors  whose  armatures  have  small 
moments  of  inertia.  As  the  speed  increases  the  resistance 
can  be  lessened  without  allowing  too  severe  a  current  to 
flow,  and  when  full  speed  is  obtained  the  resistance  must 
all  be  cut  out  to  avoid  loss.  In  order  that  counter  E.M.F. 
may  be  generated  from  the  start,  the  shunt  field  circuit 
must  first  of  all  be  closed.  These  ends  are  obtained  by  the 
use  of  a  starting-box  or  rheostat,  the  wiring  of  the  ordinary 
type  of  which  is  shown  in  Fig. 
131.  Its  main  feature  is  a  con- 
tact arm  pivoted  at  its  center, 
and  revolving  through  almost 
1 80°,  making  various  contacts. 
This  arm  is  connected  to  one 
terminal  of  the  supply  as  shown. 
As  it  is  slowly  turned  on,  one 
end  of  it  first  makes  a  connection 
which  completes  the  shunt  field 
circuit.  Then  the  other  end 
makes  a  contact  which  closes 
the  armature  and  series  coils 
through  the  maximum  resistance 
of  the  starting-box.  As  the 
speed  increases,  the  revolving  p^  ,3, 


Shunt 


DYNAMO   ELECTRIC   MACHINERY. 


arm  is  made  to  cut  out  the  resistance,  piece  by  piece,  until 
it  is  finally  all  out  of  the  circuit  and  the  machine  is  run- 
ning independent  of  the  starting  rheostat. 

Fig.    132    shows   such  a   starting-box  as   made  by   the 
Crocker   Wheeler    Company.      The  brass  contact    points 

and  the  arm  are  mounted 
on  a  slate  slab,  which 
serves  as  the  top  of  an 
open-work  cast-iron  box 
which  contains  the  resist- 
ances in  the  form  of  spiral 
coils  of  bare  wire.  The 
wire  is  generally  either  of 
German  silver  or  of  some 
one  of  the  special  iron 
alloy  resistance  materials. 
A  shunt  motor  may 
have  its  armature  coils 
destroyed  by  an  excessive 
rush  of  current  resulting  from  a  dropping  or  ceasing  alto- 
gether of  the  supply  voltage  followed  by  a  sudden  renewal 
after  the  speed  of  the  armature  has  fallen.  These  condi- 
tions may  arise  through  accidents  to  mains  or  because  of  a 
too  heavy  load  on  mains  of  insufficient  cross-section.  An 
armature  may  also  be  burned  out  by  an  excessive  cur- 
rent due  to  overloading  the  motor.  The  resulting  lower- 
ing of  its  speed  is  accompanied  by  a  corresponding  lowering 
of  the  counter  E.M.F.  Again,  a  too  high  supply  voltage, 
which  might  result  from  some  cross  or  other  accident  might 
cause  a  destructive  rush  of  current.  To  meet  these  condi- 
tions, starting  rheostats  are  often  made  with  attachments 
for  opening  the  circuit  on  no  voltage  or  low  voltage,  and 


Fig.  132. 


MOTORS. 


I/I 


Release  Magnet) 


Fig.  133- 


Fig.  134. 


172 


DYNAMO   ELECTRIC    MACHINERY. 


others  with  attachments  for  opening  the  circuit  on  overload. 
Some  have  both  attachments,  but  it  is  modern  practice  to 
remove  the  overload  attachment  from  the  starting-box  and 
put  it  on  the  switchboard.  Fig.  133  is  a  diagram  of  the 


Armature 


Details  of  Release  Magnet 


e     e 


Fig.  135- 

wiring  of  a  starting  rheostat  for  a  shunt  motor  with  auto- 
matic release  and  low-voltage  attachment.  Fig.  1 34  gives 
a  front  view  of  this  same  instrument.  When  the  starting- 


MOTORS. 


173 


handle  is  placed  in  the  "on"  position,  the  magnet  in  the 
field  circuit  holds  it  there,  although  a  spring  tends  to  throw 
it  back.  If  now,  because  of  low  voltage,  the  current  in  field 
and  magnet  becomes  weak,  the  magnet  is  no  longer  able  to 
detain  the  handle,  and  the  spring  throws  it  to  the  "  off  " 
position,  where  it  stays  until  the  motor  is  again  turned  on 
by  an  attendant. 

Figs.    1 3  5  and  1 36  show  a  view  and  a  diagram  of  the 
wiring  of  a  rheostat  with  both  release  and  overload  attach- 


Fig.  136. 

ments.  The  former  is  similar  to  the  one  just  described, 
while  the  overload  attachment  consists  of  a  magnet  in  the 
armature  circuit  which  on  overload  becomes  strong  enough 
to  attract  to  itself  a  pivoted  iron  arm  supplied  at  its  end 
with  a  device  which  short  circuits  the  field  current  around 


174 


DYNAMO   ELECTRIC   MACHINERY. 


the  release  magnet.     This   causes  the  latter   to   let 
starting-handle  drop  as  in  the  case  of  low  voltage. 


the 


93.  Characteristic  Curves  of  a  Shunt  Motor A  shunt 

motor,  having  a  small  Ra  and  a  large  Rsh,  and  having  the 
field  well  saturated,  will  give  a  fairly  constant  speed  under 
all    loads,    if  supplied    from    a    constant   pressure   circuit. 
This  is  shown  by  the  curves  in  Fig.  1 37,  which  are  from 
a  bipolar,  shunt-wound,    10  horse-power,  23<D-volt  Crocker 

Wheeler  motor. 

A  shunt  motor  when 
started  cold  on  no  load 
quickly  arrives  at  a  speed 
which  then  gradually 
rises  to  a  maximum.  The 
gradual  heating  of  the 
field  coils  increases  their 
resistance.  This  allows 
less  current  to  flow  in 
them, .  and  the  resulting 
magnetic  flux  is  less. 
Therefore  the  armature 
must  rotate  faster  to  gen- 
erate the  same  counter  E.M.F.,  as  explained  in  §  89. 

94.  Compound- Wound  Motors In  silk-mills  and  other 

textile   factories  where  any  slight  variation  in  the   speed 
affects  the  character  of  the  manufactured  product,  com- 
pound motors  give  a  satisfactorily  constant  speed.     The 
object  of  the  compounding  coils  is  to  weaken  the  flux  in 
the  armature  as  the  load  increases.     If,  at  a  given  load, 
under  the  influence  of  shunt  excitation  alone,  the  speed 


EFFICIENCY,    SPEED 
&  TORQUE   CURVES 

SIZE  IOC.  230V. 
BI-POLAR  SHdNT   MOTOR 

CROCKER-WHEELER    COMPANY 


13     14 


MOTORS.  175 

would  fall  a  certain  per  cent  of  the  speed  at  no  load,  then 
the  armature  flux  must  be  lessened  by  the  same  percentage 
in  order  to  bring  the  speed  up  to  its  original  value.  In 
calculating  the  number  of  series  turns,  account  must  be 
taken  of  the  fact  of 
magnetic  leakage, 
since  the  regulating 
coils  are  on  the  field 
magnets  and  not  on  the 
armature  direct.  Cu- 
mulatively compound- 
wound  motors  are  used  llu_  $&$ 

in   order   to    obtain    a 

large    starting   torque.  Fig.  i3s. 

The    influence    of   the 

series   coils  is  not   very   great  after  full  speed  has  been 

attained. 

95.   Hand  Speed  Regulation A  rheostat  placed  in  the 

field  circuit  of  a  shunt  motor  can  be  used  to  vary  the 
speed  of  the  motor  at  will,  as  in  Fig.  138.  An  increase 
of  the  resistance  will  decrease  the  current  in  the  field 
coils.  As  a  result  the  armature  magnetic  flux  will  decrease 
and  hence  the  speed  will  increase.  If  the  fields  be  pretty 
well  saturated,  it  will  require  a  resistance  of  some  con- 
siderable size,  say  twice  as  large  as  the  field  resistance,  to 
cut  the  current  down  enough  to  materially  reduce  the 
flux  and  increase  the  speed.  Motors  of  older  make  seldom 
had  fields  magnetized  anywhere  near  saturation.  There- 
fore they  are  very  susceptible  to  the  slightest  change  of 
resistance  in  their  field  circuits.  If  the  demagnetizing 
armature  ampere  turns  be  large,  it  is  possible  for  a  motor 


DYNAMO   ELECTRIC    MACHINERY. 


Series  ff. 


to  increase  its  speed  under  increase  of  load.     This  is  due 
to  the  decrease  of  armature  flux. 

96.   Speed  Regulation    by  Series    Resistances. The 

speed  of  a  motor  on  a  constant  pressure  circuit  can  easily 

be  varied  over  a  wide  range, 
from  rest  to  full  speed,  by 
manipulating  a  resistance 
in  series  with  it.  The  use 
of  this  method  is  not  to 
be  advised  save  for  ex- 
perimental purposes,  since 
it  is  very  wasteful  of  en- 
ergy. The  72  R  loss  in 
the  regulating  resistance 


T 


Source 


QJMMiU 


Field  Coils 


Fig-   139- 


is  sometimes  considerably 
more  than  the  power  actually  used.  Fig.  139  shows  the 
wiring  for  this  style  of  regulation. 

97.   The  Leonard  System  of  Motor  Speed  Control This 

system  is  especially  advocated  for  use  in  operating  elevators, 
cranes,  battleship  turrets,  and  all  equipments  requiring  a 
thorough  control  of  the  speed  and  precision  of  stoppage. 
Fig.  140  shows  the  arrangement  of  such  a  system.  M  is 
a  motor  whose  field  is  separately  excited  all  the  time  from 
a  source  of  constant  potential,  E.  G  is  a  dynamo  which 
generates  power  for  the  armature  of  motor  M.  The  arma- 
ture of  the  dynamo  G  is  maintained  at  approximately  con- 
stant speed  by  the  prime  mover  S,  which  may  be  a  steam 
engine  or  a  motor  run  by  power  taken  from  the  source  E. 
The  generator  G  is  separately  excited  by  current  derived 
from  E  and  controlled  by  the  reversing  rheostat  C. 


MOTORS. 


177 


When  it  is  desired  to  start  the  motor,  the  field  of  the 
generator  is  weakly  excited  by  moving  the  controller  so 
that  a  high  resistance  is  in  circuit  with  the  field.  This 
causes  the  dynamo  to  send  current  of  low  potential  to  the 
armature  of  the  motor  M.  The  latter  then  starts  to  move 
slowly.  To  accelerate  the  speed,  more  resistance  is  cut 
out  of  the  controller.  The  pressure  of  the  current  supplied 
to  the  motor  armature  simultaneously  increases  and  with 
it  the  motor's  speed.  Since  the  power  represented  by 
the  current  required  to  excite  the  field  of  G  is  at  most 


•dDJ 


Fig.  140. 

but  a  small  fraction  of  the  useful  power  given  out  by  the 
motor,  the  72  R  loss  in  the  resistance  C  is  very  much  less 
than  would  be  the  loss  in  a  series  regulating  resistance  as 
described  in  the  last  section.  It  is  claimed  that  the  extra 
first  cost  of  this  system  is  offset  by  the  decreased  cost  of 
repairs,  since  violent  stresses  and  bad  sparking  are  avoided. 


DYNAMO   ELECTRIC   MACHINERY. 


98.  Slow-Speed  Motors.  —  It  is  a  practical  advantage  to 
have  a  motor  connected  directly  to  the  machine  it  is  to 
operate,  without  the  intervention  of  belting  or  reducing 
gears.  Slow  speed  is  also  of  advantage  where  absence  of 
jarring  is  desired  or  where  many  stops  and  starts  are  to 
be  made.  Slow  speed  can  always  be  obtained  from  an 
electric  motor ;  but  it  is  generally  expensive,  since  the 
natural  speed  of  motors  as  well  as  of  dynamos  is  high.  In- 
crease of  magnetic  flux  and  increase  of  armature  diameter 
is  necessary  to  obtain  slow  speed.  The  increase  of  ma- 


::: P 


Fig.  141. 


terial  increases  both  the  first  cost  and  the  losses  during 
operation. 

The  power  of  a  motor  is  its  torque  or  turning  moment 
multiplied   by  the   number   of  revolutions ;  hence  for  the 


MOTORS. 


179 


same  output  of  work,  a  machine  making  half  as  many 
revolutions  as  another  must  have  twice  the  turning  mo- 
ment. These  conditions  make  it  imperative  that  the  ma- 
terials of  construction,  both  iron  and  copper,  be  worked  at 
maximum  magnetic  and  current  densities  respectively,  in 
order  to  economize  in  first  cost  and  weight.  In  general 
the  efficiencies  of  low- 
speed  motors  do  not 
compare  favorably 
with  those  of  motors 
having  a  higher  speed. 
Fig.  141  is  a  cut  of  a 
Crocker  Wheeler  eight- 
pole,  direct  current 
motor  for  direct  connec- 
tion. It  will  furnish  2 
horse-power  at  100  rev- 
olutions per  minute,  4 
horse-power  at  200  R. 
P.M.,  and  the  quotient 
of  its  speed  per  minute 
by  its  full  load  horse- 
power is  equal  to  the 
constant  quantity  50.  Its  efficiency  increases  as  the 
speed  according  to  the  curves  shown  in  Fig.  142. 

99.  Brake  Motors.  —  For  cranes,  elevators,  and  hoists, 
where  it  is  necessary  to  hold  the  load  after  raising  it,  and 
for  looms  and  printing-presses,  where  it  is  important  to 
secure  a  sudden  and  accurate  stop  instead  of  a  gradual 
slowing  down,  it  is  desirable  to  use  motors  with  a  brake 
attachment.  A  brake  operated  by  hand  or  foot  requires 


EFFICIENCY  AT 
VARIOUS 'SPEEDS  OF 
SIZE  2-100  MOTOR. 
CROCKER-WHEELER 

ELECTRIC  CO. 
AMPERE.  N.  J. 


100   200   300 
Fig.  142. 


10J   600   600 


i8o 


DYNAMO   ELECTRIC   MACHINERY. 


careful  operation  lest  it  be  applied  too  soon  and  injure  the 
machine,  or  too  late  and  allow  the  load  to  fall  some  ;  hence 
an  automatic  brake  is  desirable. 

Fig.  143  shows  the  construction  used  by  the  Crocker 
Wheeler  Company.  One  of  the  pole  pieces  is  pivoted  at 
its  base,  and  thus  has  a  slight  motion  to  or  from  the  arma- 
ture. It  is  normally  held  from  the  armature  by  a  heavy 
coil  spring,  and  in  this  position  tightens  the  brake  band. 
The  moment  that  current  is  allowed  to  pass  through  the 
field  coils,  the  poles  attract  each  other,  overcoming  the 


OFF 


Fig.  143. 


resistance  of  the  spring,  and  the  brake  band  is  thus  loosened. 
The  spring  and  band  may  be  adjusted  to  allow  a  few  revo- 
lutions before  stopping,  or  the  armature  may  be  clamped 
the  instant  current  is  turned  off.  In  the  latter  case,  if 
connected  to  heavy  machinery,  shafting  or  gearing  may  be 
broken. 

Strap  brakes  are  cumulative  in  their  action  ;  the  friction 
on  the  free  end  of  the  brake  against  the  drum  tightens 
the  whole  brake,  thus  increasing  its  effect.  This  action  is 


MOTORS. 


181 


only  obtained  when  the  motion  of  the  drum  is  away  from 

the  fixed  end.     To  obtain  powerful  brake  action,  therefore, 

on  motors  that  run  either  way, 

as  in  elevator  motors,  a  reversing 

brake  is  employed.     This  is  op- 
erated by  the  movement  of  one 

pole  piece  as  before,  but  the  ends 

of  the  brake  band  are  attached 

to  a  system  of  links  and  levers 

so  that  either  end  may  become 

the  fixed  end.    This  construction 

is  shown  diagrammaticallyin  Fig. 

1 44.    When  the  brake  is  applied, 

the   friction    causes    the   whole 

band  to  follow  the  drum  until  the  sliding  link  attached  to 

one  or  the  other  end  of  the  band  is  held  by  the  stud.    The 

other,  or  free,  end  of 
the  band,  is  tightened 
by  the  pull  of  the  lev- 
ers on  one  of  the 
smaller  straps  attach- 
ed to  the  brake  band 
as  shown.  Fig.  145 
shows  a  one  horse- 
power Crocker  Wheel- 
er motor  fitted  with 
reversing  brake. 

In    multipolar   ma- 
chines   it    is   imprac- 
ticable  to    employ   a 
moving  piece,  and  in 
I45.  large     bipolar     ma- 


182 


DYNAMO   ELECTRIC    MACHINERY. 


chines  it  is  undesirable  to  interrupt  the  magnetic  circuit 
by  a  pivot  joint ;  hence  a  solenoid  brake  is  employed.  This 
is  simply  a  spring  actuated  friction  brake  kept  norm- 
ally in  engagement.  On  current  being  supplied  to  the 
machine  a  solenoid  acts  to  release  the  brake.  The  opera- 
tion of  this  type  is  clearly  seen  by  inspecting  Fig.  146. 


Fig.  146. 

An  objection  to  this  type  of  automatic  brake  is  that  it 
consumes  electrical  energy  all  the  time  that  the  machine  is 
in  motion. 

loo.  Recording  Meters. — The  recording  watt-hour  meter, 
Fig.  147,  is  coming  into  extensive  use,  both  as  a  station  in- 
strument and  as  a  measurer  of  the  quantity  of  energy 
supplied  to  individual  consumers.  It  is  a  very  delicately 
adjusted  compound-wound  motor,  having  no  iron  in  its 
magnetic  circuit.  When  a  current  flows,  the  time  integral 


MOTORS. 


183 


of  the  watts  or  power  is  registered,  by  means  of  a  train  of 
wheels  operated  by  the  armature,  on  a  dial  similar  to  that 
of  a  gas-meter.  The  connec- 
tions for  such  a  meter  are 
shown  in  Fig.  148.  The  ar- 
mature is  connected  in  series 
with  a  high  resistance  across 
the  service  wires ;  hence  the 
current  flowing  in  the  arma- 
ture is  proportional  to  the 
volts  of  the  supply.  The 
field  coils  are  in  series  with 
the  service,  giving  a  field 
strength  proportional  to  the 
current,  and  the  motor  effort 
is  proportional  to  the  product  of  the  two  or  to  the  watts 
supplied.  The  shunt  field  coils  are  added  to  compensate 
for  the  friction  of  the  moving  parts.  Since  a  small  cur- 
rent is  always  flowing  in  the  armature,  as  well  as  in  the 


Fig.  147. 


Fig.  148. 


shunt  field  coils,  the  motor  is  always  slightly  excited,  and 
by  regulating  the  number   of  shunt  turns  the  amount  of 


184  DYNAMO   ELECTRIC   MACHINERY. 

this  excitation  is  adjusted  so  that  at  no  load  on  the  service 
wires  the  armature  almost,  but  not  quite,  moves.  If  it 
were  not  for  this  constant  excitation,  a  small,  though 
continuous,  current  could  be  drawn  off  the  mains  without 
operating  the  recording  mechanism  because  of  its  friction. 
To  control  the  speed  a  copper  disk  is  mounted  on  the 
armature  shaft  and  between  the  poles  of  two  or  more 
adjustable  and  permanent  horseshoe  magnets.  When  the 
armature  revolves,  Foucault  currents  are  set  up  in  this 
plate,  and  cause  the  proper  retardation.  By  moving  the 
poles  of  these  horseshoe  magnets  from  the  center  to  the 
circumference  of  the  disk,  a  variation  of  about  16  per  cent 
in  the  speed  for  a  given  watt  consumption  can  be  effected. 
Advantage  is  taken  of  this  fact  in  adjusting  the  instru- 
ments. The  more  important  bearings  are  constructed  of 
jewels,  such  as  are  used  in  watches,  and  the  whole  machine 
is  carefully  protected  from  dust.  When  the  instrument 
is  in  a  position  where  it  is  subject  to  jars  or  vibrations 
that  reduce  the  friction  of  standing  to  such  a  point  that 
the  constant  excitation  causes  the  armature  to  revolve  a 
little,  the  machine  is  said  to  "creep."  The  remedy  is 
to  mount  on  a  rubber  or  other  non -vibrating'  base,  or  to 
reduce  the  number  of  shunt  field  turns. 


SERIES    MOTORS.  185 


CHAPTER  X. 

SERIES   MOTORS. 

101 .  Series  Motors.  —  When  subjected  to  a  heavy  load  on 
starting,  that  is  when  there  is  a  heavy  current  at  a  very  low 
speed,  a  series-wound  machine  is  far  superior  to  one  that  is 
shunt-wound.    For  work  that  requires  good  effort  at  widely 
different  speeds  the  series  motor  is  particularly  adapted. 
For  this  reason  series-wound  machines  are  used  on  electric 
railways,  for  crane  motors,  for  ammunition  and  other  hoists, 
for  mill  motors,  and  in  all  other  places  where  a  good  effort 
is  required  at  a  varying  speeds.      A  series-wound  machine 
can  be  used  on  either  a  constant  current  circuit  or  on  a  con- 
stant potential  circuit  ;  but  a  series  motor  is  seldom  run  on 
a  constant  potential  circuit    unless  it   is  directly  or  very 
solidly  coupled  with  its  load,  as  in  the  case,  for  instance, 
of   a   railroad  motor.     If  connected    by  means  of  a  belt, 
and  if  the  belt  should  break  off  or  slip  off,  the  motor  would 
race  and  damage  might  result.     This   difficulty  does  not 
present  itself  when  series  motors  are  used  on  a  constant 
current  circuit. 

102.  Series  Motors  on  Constant  Potential  Circuits.  —  As 
in  the  case    of   a    shunt    motor,  on    a    constant  pressure 
circuit,  the  armature  speed  of  a  series  motor  will  increase 
until  it  reaches  a  value  where  the  counter  E.M.F.   cuts 
down  the  armature  current  to  such  a  point  that  the  total 


1 86 


DYNAMO   ELECTRIC   MACHINERY. 


electric  power,  (IE),  received  by  the  motor,  is  equal  to  the 
sum  of  the  fixed  losses,  the  variable  losses,  and  the  useful 
mechanical  power.  With  a  shunt-wound  motor,  a  very 
small  variation  of  speed  is  sufficient  to  compensate  for  a 
wide  variation  of  load.  A  series  motor  tends  to  increase 
its  speed  on  removal  of  the  load,  as  in  the  case  with  shunt 
motors.  It  in  this  manner  increases  the  counter  E.M.F. 
The  resulting  decrease  of  current  results,  however,  in  a 

weakening  of  the  field, 
and  as  a  consequence  ad- 
ditional speed  is  required 
to  maintain  the  E.M.F. 
Thus  a  small  change  in 
load  results  in  a  wide 
change  of  speed  in  a  series 
400  g  motor.  For  a  series- 

o 

300  §     wound    mill     motor,    the 

Ul 

200  1  relations  which  exist  be- 
tween speed,  current,  and 
useful  torque  (turning  mo- 
ment) are  shown  in  Fig. 
149.  There  is  also  given 
a  curve  of  the  efficiency  of  the  machine  including  gear- 
ing. It  is  evident  that  if,  while  the  motor  is  at  rest, 
the  circuit  be  closed,  an  enormous  rush  of  current  would 
occur,  giving  an  enormous  torque.  Destructive  heating 
and  sparking  would  probably  result.  To  prevent  damage 
it  is  therefore  necessary,  in  the  operation  of  these  motors, 
to  insert  a  series  resistance  at  the  start  which  may  be  cut 
out  after  the  speed  has  risen  enough  to  give  a  sufficient 
counter  E.M.F.  In  practice  controllers  are  used  as  de- 
scribed later. 


EFFICIENCY,   SPEED  AND 
TORQUE  CURVES 

SIZE  14  280  V. 
MILL  MOTOR  SERIES  WOUND 
CROCKER-WHEELER  COMPANY 


10    SO    30    40    50     60    70 
j?ig.  149. 


SERIES   MOTORS.  187 

103.  Railroad  Motors Experience  has  shown  that 

series  motors  operating  on  a  constant  potential  circuit  of 
550  volts,  furnish  a  very  satisfactory  motive  power  for  the 
propulsion  of  trolley  street-cars  and  electric  railroad  motor- 
cars. At  the  time  of  this  writing  there  are  nearly  two 
million  horse-power  of  street-car  motors  in  service  in  this 
country.  The  railway  motor  has  been  developed  to  a  high 
degree  of  perfection  during  recent  years,  and  is  reasonably 
well  fitted  to  meet  the  many  requirements  that  are  found 
in  this  service.  A  railway  motor  must  be  mechanically 
strong  to  withstand  the  excessive  hammering  to  which  it 
will  be  subjected  when  in  service.  Rough  tracks  and  bad 
switches  are  usual  in  trolley  road  beds.  When  satisfactor- 
ily geared  to  the  wheel  axle,  the  motor  can  be  suspended 
by  springs  on  one  side  only,  the  other  side  being  of  neces- 
sity mounted  directly  on  the  axle.  Railway  motors  are 
also  subject  to  abuse  at  the  hands  of  the  motormen.  The 
series  resistance  is  often  cut  out  rapidly  before  the  car  has 
an  opportunity  to  accelerate.  As  a  result  there  is  an  enor- 
mous current  and  torque  with  little  speed.  This  severely 
strains  the  motor  and  is  particularly  liable  to  disturb  the 
armature  windings.  The  motor  must  be  either  weatherproof 
of  itself  or  incased  in  a  weatherproof  shell,  because  of  the 
mud,  the  water  and  the  slush  through  which  cars  must 
often  run.  Furthermore  a  railway  motor  should  permit  of 
quick,  convenient,  and  accurate  alignment  of  parts  and 
adjustment  of  the  intermediate  driving  mechanism. 

The  method  of  suspending  the  motors  from  the  trucks 
is  a  matter  of  considerable  importance.  In  practice  four 
styles  of  suspension  are  used,  viz.,  the  side  bar,  the  cradle, 
the  nose,  and  the  yoke  suspensions.  In  every  case  one 
end  of  the  motor  frame  contains  bearings  which  run  on 


188  DYNAMO   ELECTRIC   MACHINERY. 


SERIES    MOTORS. 


189 


190 


DYNAMO   ELECTRIC   MACHINERY. 


the  wheel  axle  and  keep  the  pitch  circle  of  the  armature 
shaft  pinion  always  tangent  to  the  pitch  circle  of  the  gear 
which  is  mounted  on  the  axle.  The  side-bar  suspension, 
shown  in  Fig.  150,  consists  of  two  parallel  side-bars  which 
are  mounted  on  the  truck  through  heavy  springs,  and 
which  support  the  motor  in  the  line  of  its  center  of  gravity. 
The  motor-axle  bearings  are  thus  relieved  of  the  weight  of 
the  motor,  and  the  latter  is  held  without  undue  strains. 


Fig.  i53. 

The  cradle  suspension,  Fig.  151,  is  very  similar  to  the  side 
bar,  the  difference  being  that  the  two  side  bars  are  replaced 
by  one  U-shaped  piece.  This,  at  its  curved  end,  is  mounted 
flexibly  on  a  part  of  the  truck  frame  which  in  turn  is 
mounted  on  the  truck  through  springs.  The  nose  suspen- 
sion, Fig.  152,  does  not  hold  the  machine  at  its  center  of 
gravity,  but  part  of  the  weight  is  thrown  on  the  motor- 
axle  bearings.  The  rest  is  suspended  from  a  spring- 
mounted  member  of  the  truck  by  a  link,  bolted  to  a  "  nose" 


SERIES   MOTORS. 


191 


cast  in  the  motor  frame.  The  yoke  suspension,  which  is 
the  least  flexible  of  all,  differs  from  the  nose  suspension  in 
that  the  link  is  dispensed  with  and  the  spring-supported 
member  of  the  truck  is  bolted  rigidly  to  the  motor  frame. 
The  cradle-suspension  type  is  advocated  by  the  Westing- 
house  Company,  the  yoke  or  nose  by  the  General  Electric 
Company.  The  size  or  style  of  truck  frequently  requires  a 
particular  type  of  suspension. 


Fig.  154- 

A  GE-6/  railway  motor,  made  by  the  General  Electric 
Company,  is  shown  in  Figs.  153  and  154.  This  machine 
will  develop  38  horse-power  when  operated  on  a  5<DO-volt 
circuit  without  heating  more  than  75°  C.  above  the  sur- 
rounding atmosphere  after  one  hour's  run.  The  magnet 
frame  is  hexagonal,  with  rounded  corners,  and  is  cast  in 
two  pieces  from  soft  steel  of  high  permeability.  The 
parts  are  hinged  together  so  that  the  lower  part  may  be 


192 


DYNAMO   ELECTRIC   MACHINERY. 


swung  down  for  inspection  or  repairs  (Fig.  155).  The  up- 
per part  has  cast  on  it  two  lugs,  shown  clearly  in  Fig.  153, 
pierced  with  two  holes  each  for  bolting  to  the  yoke.  Nose 
suspension  can,  however,  be  substituted.  A  covered  opening 
over  the  commutator  permits  removal  of  the  brushes  with- 
out disturbing  the  rest  of  the  machine.  The  bearings, 
both  for  armature  shaft  and  for  axle,  consist  of  cast-iron 


Fig.  155- 

rings  or  shells,  with  Babbitt  metal  swaged  into  them,  and 
are  arranged  for  lubrication  by  both  oil  and  grease.  The 
oil  is  supplied  to  the  shafts  by  felt  wicks  leading  from  oil- 
wells.  The  grease  is  fed  through  a  slotted  opening  in  the 
top  of  each  bearing  from  a  grease-box  directly  over  each 
bearing,  and  means  is  provided  for  the  passage  of  the 
lubricant  from  the  bearing  after  it  has  been  used.  The 
armature  bearings  are  3"  x  8"  at  the  pinion  end  and  2f"  x 


SERIES   MOTORS.  193 

6\"  at  the  commutator  end.  The  motor-axle  bearings  are 
each  8"  long.  The  pole  pieces  are  built  of  soft  iron  lam- 
inations, riveted  together,  and  are  securely  bolted  into 
place  on  the  magnet  frame.  The  coils  are  spool  wound, 
and  are  held  in  place  by  steel  flanges.  These  magnetically 
imperfect  constructions  are  rendered  necessary  by  the 
severe  service  the  machine  is  expected  to  stand. 

The  armature  is  built  up  of  thin,  soft  iron  laminations, 
japanned,  and  keyed  to  the  shaft.  At  each  end  is  a  cast- 
iron  head,  also  keyed  to  the  shaft.  The  core  is  hollow, 
ventilation  being  effected  by  air  which  enters  at  the  pinion 
end  and  passes  out  through  ventilating  ducts  left  in  the 
laminations.  The  winding  is  of  the  series  drum  type,  1 1 1 
coils  being  used,  which  are  connected  to  a  commutator  of 
1 1 1  segments.  The  number  of  turns  to  a  coil  depends 
upon  the  class  of  service  the  motor  is  to  render.  The 
coils  are  made  up  of  sets  of  three,  each  set  being  sepa- 
rately insulated  before  being  placed  in  the  slots.  The 
coils  are  firmly  secured  in  place  by  tinned  steel  wire  bands 
held  by  chips  and  soldered  together.  Where  the  windings 
cross  the  ends  of  the  core,  they  are  protected  by  canvas. 
On  the  pinion  end,  a  projecting  flange  protects  the  wind- 
ings from  injury  by  careless  handling.  The  brushes  slide 
radially  in  finished  ways  in  a  brass  brush  holder,  and 
are  held  in  contact  by  independent  pressure  fingers.  All 
the  leads  to  the  motor  pass  through  rubber-bushed  holes 
in  the  front  of  the  magnet  frame.  The  pinion  has  a  taper 
fit  on  the  armature  shaft.  It,  as  also  the  gear  on  the  axle, 
is  made  of  steel,  and  has  teeth  of  4^"  face  and  3"  pitch. 
When  mounted  properly  on  a  truck  with  the  ordinary  33- 
inch  wheels,  there  is  4-J-"  clearance  between  the  bottom  of 
the  motor  and  the  top  of  the  rails.  The  shapes  of  the 


194  DYNAMO   ELECTRIC   MACHINERY. 

different  parts  of  this  motor  are  well  shown  in  the  exploded 
Fig.  156. 

A  motor  for  railway  service,  very  similar  in  design  to 
the  one  just  described,  is  No.  49,  made  by  the  Westing- 
house  Electric  and  Manufacturing  Company.  This  motor, 
shown  in  Fig.  157,  has  a  weather-proof  cast -steel  frame, 
hinged  to  open  in  a  horizontal  plane  through  its  center, 
and  having  a  hand-hole  above  and  one  below  the  com- 
mutator. The  upper  half  is  cast  with  lugs  for  side-bar  or 
cradle  suspension,  and  also  with  a  lug  for  nose  suspension. 
The  pole  pieces  are  of  laminated  soft  iron,  are  four  in 
number,  and  are  secured  to  the  frame  by  having  the  latter 
cast  around  them.  The  lathe-wound  field  coils  are  secured 
on  the  pole  pieces  by  brass  castings,  which  are  bolted  to 
the  frame. 

The  armature  is  of  the  slotted  drum  type,  having  a 
laminated  core  with  three  ventilation  passages  parallel  to 
the  shaft.  The  coils  are  wound  on  formers,  insulated  in 
sets  of  two,  and  then  applied  to  the  core.  This  armature 
is  constructed  as  light  and  as  small  in  diameter  as  is  prac- 
ticable, for  the  double  reason  of  decreased  centrifugal 
strain  on  the  armature  coils  and  decreased  wear  on  the 
parts  in  stopping  the  car.  When  the  motor  is  started, 
energy  is  stored  in  the  armature  and  other  revolving  parts, 
as  in  a  fly-wheel ;  and  when  the  car  is  stopped,  this  energy 
is  wasted,  and  causes  wear  and  tear  on  the  pinions  and 
bearings.  In  street-car  service,  where  stops  are  frequent, 
this  loss  and  this  wear  is  by  no  means  inconsiderable. 
Hence  the  armature  of  a  street-railway  motor  should  not 
be  built  with  a  great  fly-wheel  capacity.  The  high  speed 
of  car-motor  armatures  makes  the  operating  expenses  for 
car  acceleration  and  retardation  considerable. 


SERIES  MOTORS. 


195 


196  DYNAMO   ELECTRIC   MACHINERY. 


SERIES    MOTORS.  197 

104.  Controllers.  —  It  is  general  practice  to  equip  each 
trolley  car  with  at  least  two  motors,  and  to  regulate 
the  speed  of  the  car  in  the  following  manner  :  First,  the 
two  motors  and  a  resistance  are  connected  in  series.  The 
resistance  is  then  cut  out  step  by  step  until  the  two  motors 
are  operating  in  series  on  500  volts.  Since,  with  all  the 


MOTOR  1   .  MOTOR  2. 

NOTCH   1     7'ROUE*         R1  R2  R3  ARMA'       F'E<-«>  ARMA.       FIELD 

^AW  —  w,  —  m  —  ow—  ow^CK 


RUNNING). 

NOTCH  * 


RUNNING), 
NOTCH  I 


^^A^WVF^VV^ 


NOTCH  7 


RUNNING 


UNNING)™ 

NOTCH  i* 


Fig.  158. 


resistance  cut  out,  there  is  no  unnecessary  I^R  loss,  this  is 
called  a  running  connection,  and  the  controlling  mechanism 
is  said  to  be  upon  a  running  point.  To  further  increase 
the  speed,  the  motors  are  placed  in  parallel  with  a  resist- 
ance in  series  with  both.  This  resistance  is  then  cut  out 
step  by  step  until  the  motors  are  each  operating  on  500 
volts.  This,  again,  constitutes  a  running  connection.  A 
further  change  is  sometimes  effected  by  placing  a  small 


198  DYNAMO   ELECTRIC   MACHINERY. 

resistance  in  shunt  with  the  fields  when  all  the  series  re- 
sistance is  out.  This  reduces  the  field  flux,  and  causes  a 
higher  armature  speed  to  maintain  the  necessary  counter 
E.M.F.  A  car  governed  in  this  way  has  four  running 
connections.  On  heavy  cars,  such  as  are  used  in  elevated 
railway  or  inter-urban  service,  four  motors  are  used  on 
each  car.  In  this  case,  the  motors  are  governed  in  two 
series-parallel  combinations,  as  if  there  were  two  separate 
cars  governed  by  one  controller.  The  connections  for  a 
two-motor  car  having  nine  speeds,  a  three-part  series  re- 
sistance, and  a  field-shunt  resistance,  are  shown  diagram- 
matically  in  Fig.  158. 

The  different  connections  are  made  by  a  motorman,  who 
operates  a  handle  on  top  of  a  controller.  Each  different 
combination  is  called  a  point  or  a  notch.  A  pointer  affixed 
to  the  controller  handle  indicates  at  what  notch  the  car  is 
running.  Running  points  are  indicated  on  the  controller 
top  by  longer  marks  than  the  resistance  points.  A  con- 
troller is  almost  invariably  placed  at  each  end  of  the  car. 

Fig.  159  shows  the  interior  of  a  General  Electric 
Company's  k-io  series  parallel  controller.  The  wires 
from  the  trolley,  from  the  fields,  from  the  armature,  and 
from  the  different  terminals  of  the  series  and  shunt  re- 
sistances are  brought  up  under  the  car  to  terminals  on  a 
connecting-board  in  the  bottom  of  the  controller.  On 
this  connecting-board  there  are  also  switches,  one  for  each 
motor.  These  enable  one  to  cut  out  an  injured  motor 
without  interfering  with  the  operation  of  the  other  motor 
or  motors.  From  the  connecting-board  conductors  are 
run  to  terminals,  called  fingers  or  wipes.  Mounted  on  an 
insulating  cylinder,  which  may  be  revolved  by  the  con- 
troller handle,  are  insulated  contact  pieces,  which  at  various 


SERIES   MOTORS. 


199 


angular  positions  of  the  cylinder  make  electrical  connec- 
tions between  various  wipes,  and  give  the  proper  con- 
nections for  the  various  "points"  or  " notches."  A 


Fig.  159. 

smaller  cylinder  connected  to  a  reversing-lever,  is  situated 
to  the  right  of  the  main  cylinder.  This  has  contact  pieces 
which  are  arranged  so  as  to  enable  the  motorman  to  re- 
verse the  direction  of  rotation  of  both  motors  or  to  cut 
them  out  entirely.  Interlocking  devices  are  supplied  so 


200  DYNAMO  ELECTRIC  MACHINERY. 

that  the  reversing  handle  cannot  be  moved  unless  the  con- 
trolling handle  is  in  such  a  position  that  connection  with 
the  trolley  is  broken.  The  controlling  handle  also  cannot 
be  moved,  if  the  reversing  handle  is  not  properly  set  either 
to  go  forward  or  to  go  backward.  The  reversing  handle 
cannot  be  removed  from  the  controller,  save  when  the 
smaller  cylinder  is  in  the  position  that  cuts  out  both  motors. 
As  serious  arcs  are  liable  to  develop  upon  breaking  a 
circuit  of  500  volts,  the  contact  pieces  and  wipes  are  sepa- 
rated from  adjacent  ones  by  strips  of  insulating  material 
which  are  fastened  to  the  inside  of  the  controller  cover, 
and  which  fold  into  place  when  the  cover  is  closed.  These 
are  to  be  seen  at  the  right  of  the  figure.  The  power 
should  never  be  turned  off  by  a  slow  reverse  movement  of 
the  controller  handle,  as  destructive  arcs  are  liable  to 
occur  upon  a  slow  break.  To  lessen  the  speed  of  a  car, 
the  power  should  be  completely  and  suddenly  shut  off. 
Before  the  car  has  slackened  its  speed  too  much  the  con- 
troller handle  can  be  brought  up  to  the  proper  point.  The 
arcs,  which  form  upon  disconnection  at  the  fingers,  are 
pretty  effectively  blown  out  by  the  field  of  an  electro- 
magnet whose  coil  is  above  the  connecting-board  at  the 
right. 

105.  Motors  For  Automobiles.  —  For  electric  automo- 
biles the  series-wound  motor  is  invariably  employed.  A 
storage  battery  of  40  or  44  cells  is  the  customary  source 
of  power  for  these  motors.  The  use  of  these  cells  affords 
a  convenient  and  economical  means  of  speed  control.  In 
the  case  of  a  single  motor,  for  the  first  controller  notch, 
the  cells  may  be  connected  in  four-series  groups  of  10  or 
II  each,  giving  about  22  volts,  the  four  groups  being  con- 


SERIES   MOTORS.  2OI 

nected  in  parallel.  Other  notches  would  correspond  to 
other  series  parallel  combinations,  and  finally  the  last  and 
highest  speed  notch  would  correspond  to  a  connection  of 
all  the  cells  in  series.  By  this  arrangement  one  cell  is 
used  just  as  much  as  any  other,  and  they  are  discharged  at 
equal  rates.  As  the  voltage  supplied  to  the  motor  is 
varied  without  recourse  to  a  series  regulating  resistance, 
there  is  no  useless  I^R  loss  in  starting  or  running  at  less 
than  full  speed.  Often  a  series  parallel  control  is  employed 
when  two  motors  are  used.  It  is  also  common  to  use  two 
37^  volt  motors  connected  permanently  in  series  and  con- 
trolled as  one  motor. 

The  advantage  of  using  two  motors  on  an  automobile  is 
that  each  may  drive  a  wheel,  allowing  independent  rota- 
tion on  turning  curves,  while  if  one  motor  only  is  used 
some  form  of  differential  gear  must  be  employed  to  allow 
for  sharp  turns.  But  the  efficiency  of  one  motor  is  in 
general  greater  than  the  efficiency  of  two  motors  of  half 
the  power,  and  the  gain  in  efficiency  by  using  one  motor 
more  than  balances  the  cost  and  complication  of  a  differ- 
ential gear. 

The  question  of  efficiency  in  these  motors  is  of  great 
importance,  for  practice  has  shown  that  it  is  profitable  to 
purchase  I  per  cent  efficiency,  even  at  the  cost  of  10  per 
cent  increase  of  motor  weight.  This  is  because  the  ratio 
of  the  battery  weight  to  the  motor  weight  is  such  that  a 
decrease  of  i  per  cent  in  the  capacity  of  the  battery  re- 
duces its  weight  more  than  10  per  cent  of  the  motor 
weight.  Since  lightness  is  a  prime  object,  only  the  very 
best  materials  can  enter  into  the  construction  of  a  suc- 
cessful automobile  motor.  The  magnetic  circuit  must  be 
of  material  of  the  highest  permeability.  Ball  bearings  are 


202  DYNAMO   ELECTRIC   MACHINERY. 

not  infrequently  used  in  the  shaft  bearings,  but  their  lia- 
bility to  wear  and  the  consequent  regrinding  is  an  objec- 
tion. 

It  is  general  practice  to  rate  these  motors  at  75  volts,  or 
37 \  volts  if  two  are  used.  Since  40  or  44  cells  of  battery  in 
series  can  fall  to  75  volts  without  injury,  this  is  the  lowest 
pressure  on  which  the  motors  will  be  expected  to  run  for 
any  length  of  time  at  full  speed.  Hence  this  voltage  is 
used  as  the  basis  for  rating.  For  the  best  motors  the 
rating  is  for  a  temperature  rise  of  50°  or  60°  C.  on  an  in- 
definite run.  A  motor  so  rated  will  carry  100  per  cent 
overload  for  half  an  hour,  150  per  cent  for  ten  minutes,  and 
a  momentary  overload  of  400  or  500  without  overheating 
or  damage  to  the  insulation. 

The  battery  of  40  or  44  cells  is  well  adapted  to  automo- 
bile purposes.  It  can  conveniently  be  made  to  have  the 
required  capacity,  and  it  may  be  charged  from  any  1 1 5- 
volt  direct  current,  incandescent  lighting  circuit  with  very 
little  resistance  in  series  and  hence  a  small  I^R  loss. 

Although  the  voltage  of  these  motors  is  somewhat  low 
for  the  use  of  carbon  brushes,  the  necessity  of  reversal  of 
direction  and  the  liability  of  sparking  on  over-load  make 
their  use  desirable.  Soft  carbon  brushes  of  low  resistance 
can,  however,  be  obtained,  and  they  are  to  be  recom- 
mended. 

Fig.  1 60  illustrates  a  motor  which  is  used  on  automo- 
biles and  manufactured  by  the  Eddy  Electric  Manufacturing 
Company.  It  is  a  four-pole  machine.  The  frame  is  ring 
shape  and  made  of  cast  steel.  The  pole  pieces,  also  made 
of  cast  steel,  are  fastened  to  the  frame  by  bolts  and 
steady  pins.  The  armature  is  wound  with  formed  coils 
which  are  cross  connected,  and  therefore  require  but  two 


SERIES   MOTORS.  203 

sets  of  brushes.  These  brushes  are  made  accessible  by 
the  existence  of  a  window  in  the  end  plate.  A  pinion 
which  is  mounted  upon  the  armature  shaft  meshes  with 
an  inside  gear  placed  upon  the  wheel  of  the  vehicle.  A 
recess  in  the  exterior  of  the  magnet  frame  is  fitted  to  re- 
ceive some  part  of  the  frame  of  the  vehicle.  Clamps  for 
fastening  to  this  frame  are  provided  to  suit  the  character  of 
the  vehicle.  The  motor  is  intended  to  be  operated  on  75 
volts,  and  is  rated  at  1.6  horse-power,  at  the  speed  of  1400 


Fig.  160. 

revolutions  per  minute.  Its  weight  is  142  Ibs.,  and  it 
has  an  efficiency  of  79^  per  cent  at  full  load.  At  100  per 
cent  over-load  it  has  an  efficiency  of  76^  per  cent,  and  at 
150  per  cent  over-load  an  efficiency  of  73  per  cent. 

1 06.  Mill  Motors.  —  For  many  kinds  of  mill  work  re- 
quiring great  torque  at  low  speed,  reversibility,  and  wide 
variation  of  speed,  the  series-wound  motor  is  well  adapted. 
Since  mill  motors  are  to  be  used  in  places  where  dust, 
grit,  and  small  particles  of  metal  are  apt  to  be  floating  in 
the  air,  it  is  necessary,  to  insure  good  continuous  operation, 


204  DYNAMO   ELECTRIC   MACHINERY. 


SERIES   MOTORS. 


205 


B 


206 


DYNAMO   ELECTRIC   MACHINERY. 


that  they  be  inclosed  after  the  fashion  of  railway  motors. 
Mill  motors  differ  from  shunt-wound  machines  in  that  they 
are  capable  of  giving  a  turning-power,  when  slowed  down 
or  started  from  rest,  many  times  as  great  as  that  given 
at  full  speed. 

Fig.  161  shows  a  Crocker  Wheeler  mill  motor,  and  Fig. 
162  shows  the  same  disassembled.  It  is  a  bipolar  drum 
armature  machine,  designed  for  about  800  R.P.M.,  and 
giving  without  overheating  an  intermittent  horse-power  of 

14  or  a  continuous  horse-power  of  5. 
It  is  rated  in  this  way,  since  fre- 
quent stops  and  starts  are  expected 
in  the  use  of  such  a  motor.  The 
hotter  a  motor  gets  during  an  in- 
terval of  use  the  more  it  will  cool 
off  during  an  interval  of  rest.  Of 
course  an  inclosed  motor  such  as 
a  mill  motor  heats  up  much  more 
rapidly  and  severely  than  does  an 
open  motor  where  the  air  circulates 
around  the  fields  and  the  armature. 
Since  these  motors  are  reversi- 
ble the  brushes  can  have  no  lead. 
Sparkless  running  is  accomplished 
by  a  long  air  gap.  Being  series 
wound  the  field  increases  with  load 
and  the  speed  is  reduced  corre- 
spondingly, hence  commutation  is  readily  effected. 

These  motors  are  controlled  by  a  variable  series  resist- 
ance, the  various  connections  being  made  in  a  controller, 
such  as  is  shown  in  Fig.  163.  The  circuits  are  made  by  con- 
tact pieces  on  a  cylinder  coming  in  contact  with  fingers  or 


Fig.  163. 


SERIES    MOTORS  2O/ 

wipers  which  are  mounted  on  a  board  forming  the  back  of 
the  controller.  The  controller  illustrated  is  also  a  reverser. 
The  motor  can  be  run  in  either  direction  by  moving  the 
controller  handle  to  the  right  or  to  the  left  of  the  central 
position. 


208  DYNAMO   ELECTRIC   MACHINERY. 


CHAPTER   XI. 

DYNAMOTORS,   MOTOR-GENERATORS, 
AND   BOOSTERS. 

107.  Dynamotors.  —  A  dynamotor  is  a  transforming 
device  combining  both  motor  and  generator  action  in  one 
magnetic  field,  with  two  armatures  or  with  an  armature 
having  two  separate  windings.  They  are  generally  sup- 
plied with  a  commutator  at  each  end,  which  are  connected 
to  the  two  windings  respectively.  Either  winding  of  the 
armature  may  be  used  as  a  motor  winding,  and  the  other 
as  the  dynamo  winding.  These  machines  occupy  the  same 
position  as  regards  direct  current  practice  as  is  occupied 
by  transformers  in  alternating  current  practice.  That  is, 
they  enable  one  to  take  electrical  energy  from  a  system 
of  supply  at  one  voltage,  and  deliver  it  at  another  voltage 
to  a  circuit  where  it  is  to  be  utilized.  They  cannot,  how- 
ever, be  constructed  so  as  to  operate  with  the  same  high 
efficiency  as  a  transformer  does.  As  the  currents  in  the 
two  armatures  flow  in  opposite  directions,  and' the  machines 
are  so  designed  as  to  have  practically  the  same  number  of 
armature  ampere  turns  when  in  operation,  there  is  practi- 
cally no  armature  reaction.  The  field,  therefore,  is  not 
distorted  so  as  to  require  a  shifting  of  the  brushes,  nor  is 
there  sparking  present  as  a  result  of  a  change  of  load. 
These  machines  are  more  efficient  than  motor  generators, 
which  will  be  described  later,  as  they  have  but  a  single 


DYNAMOTORS.  209 

field.  They  cannot  be  compounded  so  as  to  yield  a  con- 
stant E.M.F.  at  the  dynamo  end.  A  cumulative  series 
coil  would  tend  to  raise  the  E.M.F.  at  the  dynamo  end, 
but  it  would  lower  the  speed  of  the  armature  as  a  motor 
by  a  corresponding  amount. 

1 08.  The  Bullock  Teaser  System.  —  Dynamotors  are 
used  extensively  by  the  Bullock  Electric  Manufacturing 
Company  in  their  so-called  Teaser  system  of  motor-speed 
control.  This  system  is  used  in  driving  large  printing- 
presses  from  supply  circuits,  which  are  at  the  same  time 
used  for  lighting  and  other  purposes.  Large  printing- 
presses  contain  very  many  sets  of  gears,  and  possess  very 
large  moments  of  inertia.  These  large  machines  require 
an  unusually  large  torque  on  the  part  of  the  motor  to  start 
them.  Sometimes  it  is  as  much  as  five  or  six  times  that 
torque  which  the  motor  is  called  upon  to  produce  at  full 
load.  Now,  the  torque  which  is  exerted  by  a  motor  is  de- 
pendent upon  the  current  which  flows  through  its  arma- 
ture, while  the  speed  at  which  this  torque  is  applied  is 
dependent  upon  the  impressed  electromotive  force.  As 
the  current,  which  is  required  to  produce  the  normal  run- 
ning torque  is  already  of  considerable  strength,  it  is  desira- 
ble that  some  direct  current  electrical  transformation  be 
employed  to  avoid  the  excessive  starting  current.  The 
Teaser  system  accomplishes  this  by  making  use  of  the 
dynamotor.  The  motor  winding  is  designed  for  five  times 
the  electromotive  force  of  the  dynamo  winding.  Its  field 
winding  is  excited  directly  from  the  supply  mains.  The 
negative  brush  of  the  motor  side  is  connected  with  the 
positive  brush  of  the  dynamo  side.  The  two  armature 
windings  are  connected  in  series  with  a  regulating  resist- 


2IO 


DYNAMO  ELECTRIC  MACHINERY. 


ance  to  the  supply  mains.  At  starting,  the  main  motor, 
which  drives  the  press  and  which  is  generally  a  cumu- 
latively compound-wound  motor,  is  supplied  with  current 
from  the  dynamo  end  of  the  dynamotor.  The  voltage 
with  which  it  is  supplied  is  somewhat  less  than  one-fifth 
that  of  the  main  supply,  depending  upon  the  magnitude 
of  the  resistance  in  series  with  the  dynamotor.  This  low 


voltage  permits  of  the  application  of  a  proper  amount  of 
torque  at  a  low  speed.  Furthermore,  the  drain  of  current 
from  the  supply  mains  is  but  about  one-fifth  that  which 
passes  through  the  main  motor.  By  manipulating  the  dy- 
namo regulating  resistance,  the  electromotive  force  sup- 
plied to  the  main  motor  is  raised,  and  with  it  the  speed. 
The  highest  speed  of  the  main  motor  which  can  be  attained 
by  this  arrangement  is  such,  that,  when  attained,  the  mo- 
tor's connections  may  be  transferred  to  the  supply  mains 


DYNAMOTORS. 


211 


AMAMAA/VV\ 


/XAAAAA/WJ 


through  another  series  regulating  resistance  without  any 
excessive  drain  of  current  from  those  mains.     The  arrange- 
ment   of   the   apparatus    is    shown  in   Fig.    165,  and  the 
amount  of   current  which  is 
taken  by  the  main  motor  as 
compared  with    the  amount 
of  current   which    is    drawn 
from  the  supply  mains  is  re- 
presented in  Fig.  164.     Re- 
gulation   of    the    resistances 
and    changes    of    connection 
are  accomplished  through  the 
aid   of    a    controller.       The 
different  speeds  are  secured 
by   the    manipulation    of    a 

single  hand-wheel  on  the  con- 
Fig.  165. 

troller,  and  thus  the  press- 
man has  at  his  command  a  means  of  manipulating  the  press 
which  is  not  complicated. 

109.  Dynamotors  for  Electro-Deposition  of  Metals. —  In 
large  electro-plating  establishments,  it  is  common  to  in- 
troduce a  dynamotor,  whose  two  armature  circuits  are 
exactly  similar,  and  under  ordinary  excitation  give  5  or  10 
volts.  The  commutators,  brushes,  collecting  devices,  and 
leads  are  of  necessity  quite  massive.  The  leads  are  gener- 
ally so  arranged  that  the  two  armatures  may  be  placed  in 
series  with  each  other,  or  in  multiple.  The  low  voltage 
of  platers  makes  it  impracticable  to  have  a  machine  self- 
exciting.  It  is  common  practice  in  cities  to  excite  these 
machines  from  no-volt  lighting  circuits,  with  a  regulating 
rheostat  whose  resistance  is  of  such  a  magnitude  as  to 


212  DYNAMO   ELECTRIC   MACHINERY. 

permit  of  the  variation  of  the  voltage  of  the  machine  over 
a  range  of  25  per  cent  of  its  full-load  value.  Fig.  166 
shows  a  dynamotor  constructed  by  the  Eddy  Electric 
Manufacturing  Company  for  the  electro-deposition  of  cop- 
per. Each  armature  winding  gives  10  volts  and  4,500 


tig. 


amperes.  It  is  designed  to  be  belt  -driven  through  a  large 
pulley  at  one  end  of  the  armature  shaft.  A  small  pulley 
upon  the  other  end  is  for  the  purpose  of  receiving  a  belt 
connected  with  a  small  separate  exciter.  The  large  split 
clamps  connected  with  the  leads  are  for  the  reception  of 
the  terminals  of  the  main  conductors. 

no.  The  Eddy  Company's  Rotary  Equalizer  ---  This  is 
a  dynamotor  having  a  single  field  which  is  excited  from  a 
22O-volt  circuit,  and  a  single  armature  core  upon  which  is 
wound  two  distinct  1  1  o-volt  armatures.  One  armature 


DYNAMOTORS. 


213 


has  its  commutator  on  one  end  of  the  shaft  and  the  other 
at  the  other  end.  The  machine  is  used  in  connection  with 
a  22O-volt  generator,  to  enable  one  to  use  it  for  supplying 
energy  to  a  three-wire,  no-volt,  incandescent  lighting  sys- 
tem. The  principle  of  its  action  can  be  seen  from  an 
inspection  of  Fig.  167.  When  the  system  is  unbalanced, 


Fig.  167. 

that  side  which  has  the  smaller  load  has  the  lesser  drop, 
and  therefore  the  higher  difference  of  potential.  The 
armature  winding  of  the  dynamotor  which  is  connected 
with  that  side  acts  as  a  motor,  runs  the  armature,  and 
causes  the  other  armature  winding  to  act  as  a  generator  in 
raising  the  pressure  of  the  heavier  loaded  side.  Obvi- 
ously the  employment  of  this  system  can,  in  some  cases, 
result  in  a  considerable  saving  of  copper. 

in.  Other  Applications  of  Dynamotors.  —  The  Crocker 
Wheeler  Company  manufactures  a  special  line  of  dyna- 
motors  (Fig.  168)  for  use  in  telegraphic  work.  The  motor 
is  designed  to  be  supplied  with  electrical  energy  from  street 
service  mains,  or  from  the  house-lighting  mains  in  the 
case  of  isolated  plants.  The  generator  end  furnishes  cur- 
rents at  a  constant  potential,  which  is  different  in  the  case 


214 


DYNAMO   ELECTRIC   MACHINERY. 


of  different  machines.  These  machines  are  designed  to 
take  the  place  of  batteries  of  a  large  number  of  gravity 
cells  such  as  were  used,  in  large  quantities,  a  few  years  ago. 
The  cost  of  operation  of  a  dynamotor  for  this  service  is 
about  one-fifth  of  what  it  is  in  the  case  of  the  gravity 
cells.  The  space  which  the  machine  occupies  is  but 


79 


Fig.  168. 

that  of  the  cells.  They  are  to  be  preferred  to  bat- 
teries also  on  the  ground  of  cleanliness.  Their  reliability, 
when  supplied  by  electric  energy  from  large  city  service 
mains  is  equal  to  that  of  the  cells.  The  same  cannot  be 
said  in  the  case  of  small  towns.  The  telephone  companies 
are  also  rapidly  adopting  the  dynamotor  for  the  purpose 
of  charging  storage  cells.  With  some  forms,  the  charging 
of  the  cells  can  go  on  continuously,  they  being  at  the  same 


DYNAMOTORS.  215 

time  used  for  telephone  purposes.  Dynamotors  also  fur- 
nish a  convenient  and  satisfactory  means  of  heating 
surgeons'  electro-cauteries.  Cautery  knives  take  from  3 
to  8  amperes  at  5  volts,  while  dome  cauteries  take  from 
1 5  to  20  amperes  at  the  same  voltage. 

112.  Motor-Generators.  — A  motor  generator  is  a  trans- 
forming device  consisting  of  two  machines,  a  motor  and 
generator,  mechanically  connected  together.  They  have 
the  advantage  over  dynamotors  in  that  the  voltage  of  the 


Fig.  169. 

dynamo  armature  can  be  made  to  assume  almost  any 
value  within  limits  by  means  of  a  resistance  placed  in 
series  with  its  field-winding  and  capable  of  variation. 
They  can  furthermore,  besides  being  separately  excited, 
be  shunt  wound  or  compound  wound.  They  are  used 
quite  extensively  in  the  Ward-Leonard  system  of  motor 
speed  control,  which  was  described  in  paragraph  97.  They 
are  also  used  for  charging  storage  batteries.  In  this  case 


2l6  DYNAMO   ELECTRIC   MACHINERY. 

they  are  almost  always  shunt  wound.  They  are  also  used 
in  electro-plating  establishments.  In  this  case  they  are 
separately  excited,  as  the  voltage  generally  employed  in 
such  places  is  too  small  to  give  satisfactory  self-excitation. 
For  general  laboratory  work  on  tests  which  require  large 
current  at  a  low  voltage  or  a  small  current  at  a  high 
voltage,  motor  generators  are  of  inestimable  value. 

113.  Boosters —  A  booster  is  a  machine  inserted  in  series 
in  a  circuit  to  change  its  voltage,  and  may  be  driven  either 
by  an  electric  motor,  or  otherwise.  In  the  former  case  it  is  a 
motor-booster.  This  machine  is  used  very  extensively  on 
Edison  three-wire  incandescent  lighting  systems  which 
supply  current  at  a  constant  potential.  Feeders  which  run 
to  feeding-points  at  a  great  distance,  if  supplied  by  current 
from  the  same  bus  bars  as  shorter  feeders,  will  have  too 
small  a  difference  of  potential  at  the  feeding-points  to  give 
satisfactory  service.  A  booster  with  its  field  and  armature 
windings  in  series  inserted  in  series  in  the  feeder  will  add 
E.M.F.  to  the  feeder  which  in  magnitude  is  proportional  to 
the  current  flowing  in  the  feeder,  that  is,  as  the  current  in- 
creases the  field  excitation  will  increase  and  with  it  the 
E.M.F.  produced  by  the  armature.  The  machine  may, 
therefore,  be  so  designed  as  to  just  compensate  for  any 
drop  which  is  due  to  the  resistance  of  the  feeders  and  to 
the  current  flowing  through  them.  As  all  the  current  of  the 
feeder  must  pass  through  the  booster  armature,  the  collect- 
ing devices  must  be  massive  and  must  be  designed  to  carry 
these  heavy  currents.  The  rating  of  a  booster  is  of  course 
determined  by  the  voltage  which  it  produces,  and  the  total 
current  which  passes  through  it  and  the  feeders.  Boosters 
are  also  used  in  the  central  stations  of  trolley  companies  to 


DYNAMOTORS.  2I/ 

raise  the  voltage  which  is  supplied  to  the  feeders  connected 
with  distant  sections  of  the  line.  They  are  also  being  in- 
troduced in  office  buildings  in  connection  with  electric 
elevator  service.  When  the  elevator  motors  are  supplied 
from  the  same  generators  as  the  lights  and  fans  in  an  office 
building  they  give  to  the  generators  what  is  called  a  lumpy 
load.  The  excessive  currents  demanded  by  the  elevator 
motors  on  starting  produce  wide  fluctuations  of  voltage  in 
the  mains.  A  booster  inserted  in  these  mains  may  be 
made  to  add  E.M.F.  to  the  mains  on  these  occasions. 


218  DYNAMO   ELECTRIC   MACHINERY. 


CHAPTER   XII. 

MANAGEMENT     OF    MACHINES. 

114.   Connections  for  Combined  Output  of  Dynamos.— 

In  general  a  dynamo  is  much  more  efficient  when  operated 
at  its  full  load  than  when  operated  at  one-half  or  one-quarter 
load.  It  is  usual  to  install  in  central  stations,  which,  as  a 
rule,  have  to  supply  different  quantities  of  electrical  energy 
at  different  times  of  the  day,  a  number  of  smaller  units 
rather  than  one  unit  large  enough  to  supply  the  total 
energy.  By  this  means  any  load  can  be  handled  by  a 
machine  or  a  number  of  machines  all  operating  at  about  their 
maximum  of  efficiency.  It  is  well,  therefore,  to  consider 
the  methods  of  combining  two  or  more  machines  on  one 
load.  The  simplest  and  most  usual  method  of  connecting 
dynamos  is  that  employed  in  incandescent  light  generating 
stations.  Here  a  number  of  constant  pressure  machines 
are  arranged  as  in  Fig.  1 70,  to  act  in  parallel  on  one  pair  of 
bus  bars.  The  figure  shows  shunt  machines  with  hand 
regulators.  The  various  external  circuits  are  connected  in 
parallel  to  the  bus  bars.  This  practice  is  frequently 
modified  by  separating  those  machines  which  supply  the 
circuits  that  deliver  at  the  more  distant  points  from  those 
that  operate  the  shorter  circuits.  This  is  because  the  main- 
taining of  a  constant  and  uniform  pressure  at  all  distribut- 
ing points  requires  a  higher  pressure  on  the  station  ends  of 
the  longer  mains  than  on  the  shorter.  When  a  machine 


MANAGEMENT   OF   MACHINES. 


219 


is  to  be  thrown  into  circuit  on  to  bus  bars  already  in  opera- 
tion, it  is  first  brought  up  to  speed,  the  field  magnetization 
is  then  adjusted  till  the  machine  gives  the  same  pressure 
as  exists  between  the  bus  bars,  and  the  main  switch  is  then 


H-BUS 


+  BUS 


closed,  which  puts  the  machine  in  circuit.  The  proper 
pressure  at  which  to  throw  in  the  new  machine  may  be 
roughly  determined  by  comparing  the  relative  brightness 
of  its  pilot  lamp  with  that  of  the  lamps  operating  on  the 
circuit. 

m  ic 

A  more  exact  way  is  to 
compare  the  readings  of  a 
volt-meter  across  the  ter- 
minals of  the  machine  with 
one  across  the  bus  bars. 
The  most  convenient  way  is 
to  use  a  "cutting-in  galvano- 
meter." Of  these  there  are 
two  forms,  the  zero  galvanometer  and  the  differential  gal- 
vanometer. The  zero  galvanometer,  shown  with  connec- 
tions in  Fig.  171,  has  a  single  coil  of  high  resistance. 
When  the  pressure  of  the  machine  is  not  exactly  that  of 
the  bus  bars  a  current  will  flow  one  way  or  the  other,  and 


Fig.  171. 


220  DYNAMO   ELECTRIC   MACHINERY. 

the  needle  will  be  correspondingly  deflected.  When  there 
is  no  deflection,  the  machine  may  be  thrown  into  circuit. 
This  instrument  is  simple  and  cheap,  but  it  requires  that 
one  terminal  of  the  machine  be  permanently  connected  to 
a  bus,  which  is  not  always  desirable.  The  differential  gal- 
vanometer, Fig.  172,  has  two  high  resistance  coils,  one  in 
shunt  across  the  bus  bars,  and  one  in  shunt  across  the 
machine  terminals. 

When  equal  pressures  are     ___^ -  -  BUS 

impressed    on    each    of    the     I 

coils,  they,  by  their  differ- 
ential action,  hold  the  needle 
in  equilibrium,  but  when  one 
coil  is  subject  to  more  pres- 
sure than  the  other  a  deflec- 
tion occurs.  This  instrument 

i  i  Fig.  172. 

is    more    costly    and    more 

complex  than  the  last,  but  it  has  the  advantage  that  a  two- 
pole  switch  may  be  used  to  cut  in  or  out  the  machine. 

When  shunt  machines  are  connected  in  parallel,  it  is 
expected  that  they  will  all  be  kept  at  the  same  pressure. 
If  they  are  not,  no  serious  damage  is  likely  to  occur,  since 
the  lower  pressure  machine  merely  fails  to  take  its  full 
share  of  the  load.  If  the  pressure  of  one  machine  falls  so 
low  that  it  is  overpowered  and  run  as  a  motor,  still  no 
damage  will  result,  save  perhaps  the  blowing  of  a  fuse, 
since  the  direction  of  rotation  for  a  shunt  machine  is  the 
same  whether  it  be  run  as  a  dynamo  or  as  a  motor.  If  it 
be  desired  to  regulate  a  number  of  machines  together  by 
one  regulator,  it  may  be  accomplished  by  bringing  the 
positive  ends  of  the  field  coils  to  one  side  of  the  regulator 
and  connecting  the  other  side  to  the  negative  bus. 


MANAGEMENT   OF   MACHINES. 


221 


Shunt  machines  may  be  operated  in  series  by  connecting 
the  positive  brush  of  one  machine  to  the  negative  brush  of 
the  next,  and  connecting  the  extreme  outside  brushes  with 
the  line  wires.  When  this  is  done  each  machine  can  be 
regulated  separately  to  generate  any  portion  of  the  pressure. 
If  it  be  desired  to  regulate  all  the  machines  thus  connected 
uniformly  and  as  a  unit,  the  field  coils  of  all  the  machines 
may  be  put  in  series  with  one  regulating  rheostat,  and 
shunted  across  the  extreme  brushes  of  the  set.  In  the 
Edison  three-wire  system  two  ii5-volt  direct-connected 
shunt  machines  are  mounted  on  one  engine  shaft.  The 
dynamos  are  connected  in  series  as  described  above,  the 
neutral  wire  being  connected  to  the  united  brushes,  as  in 
Fig.  173. 

Series-wound  dynamos  may  be  operated  in  series  with- 
out any  difficulty,  though  it  is  not  customary  to  do  so. 
Series  generators  are  used  almost  exclusively  on  constant 
current  (arc  light)  circuits,  and  it  is  usual  to  have  as  many 
machines  as  there  are  external  circuits,  each  machine  being 
of  capacity  enough  to  operate  that  circuit  alone.  A  new 

form  of  Brush  generator  supplies 
several  series  circuits  from  its 
terminals,  and  regulates  for  all 
of  them.  If  it  be  attempted  to 
operate  series  dynamos  in  paral- 
lel, the  following  difficulty  occurs : 
If  the  machines  start  with  a 
proper  distribution  of  load  among 
them  and  one  does  not  generate 

just  its  full  pressure,  then  this  one  does  not  continue  to  take 
its  full  share  of  the  load  ;  and,  since  it  is  series  wound, 
a  decrease  in  load  is  followed  by  a  decrease  in  pressure. 


Fig.  173- 


222  DYNAMO   ELECTRIC   MACHINERY. 

The  conditions  become  always  more  uneven  until  the 
machine  is  overpowered  and  it  turns  into  a  motor.  Since 
the  direction  of  rotation  of  a  series-wound  motor  is  oppo- 
site to  its  direction  when  run  as  a  dynamo,  serious  results 
may  occur.  The  only  remedy  for  this  trouble  is  to  arrange 
the  field  coils  so  that  the  magnetization  in  any  one  machine 
will  remain  the  same  as  in  the  other  machines,  even  though 
its  pressure  falls  below  that  of  the  others.  To  accomplish 
this  the  series  fields  must  all  be  placed  in  parallel.  This 
may  be  done  by  means  of  an  equalizer,  which  is  a  wire  of 
small  resistance  connected  across  one  set  of  brushes,  and 
by  placing  the  fields  in  parallel,  as  shown  in  Fig.  1 74.  Two 


Fig.  174. 


series  dynamos  may  be  run  in  parallel  without  an  equalizer 
by  resorting  to  mutual  excitation,  that  is,  by  letting  the  cur- 
rent of  one  armature  excite  the  field  of  the  other.  In  this 
case  if  the  pressure  of  one  machine  falls  and  its  load  there- 
fore decreases,  the  magnetization  of  the  other  is  reduced, 
compelling  the  first  to  maintain  its  share  of  the  load. 
Series  dynamos  are  never  operated  in  parallel  in  practice, 
but  this  discussion  is  introduced  because  of  its  application 
to  compound-wound  dynamos. 

The  use  of  compound  generators  for  constant  pressure 
circuits  is  very  common.  Since  these  have  series  coils 
they  cannot  be  run  in  parallel  without  special  arrange. 


MANAGEMENT   OF   MACHINES.  223 

ments.     It  is  usual  to  fit  the  series  coils  with  an  equalizer, 
as  in  Fig.  175.      The  desired  end  might  be  accomplished 
in  the  case  of  two  ma- 
chines  by  making  the 
series  coils  mutually  ex- 
citing. 


115.    Connections  of 
Motors    for    Combined 

Output Any  number 

of  shunt  motors  may  be  placed  in  parallel  across  mains 
of  a  constant  pressure,  and  their  operation  will  be  sat- 
isfactory whether  each  has  a  separate  load  or  whether 
they  be  connected  through  proper  ratios  to  one  shaft. 
Shunt  motors  will  operate  in  series  on  a  constant  pressure 
circuit  when  positively  connected  together ;  but  if  con- 
nected to  the  same  shaft  by  belts,  and  one  belt  slips  or 
comes  off,  that  motor  will  race,  and  rob  its  mates  of  their 
proper  portion  of  the  voltage.  This  arrangement  is  not 
common. 

Series  motors  will  operate  satisfactorily  on  constant 
pressure  circuits :  but  when  two  or  more  such  machines, 
that  are  arranged  in  parallel  on  a  constant  pressure  circuit, 
are  connected  to  one  shaft  an  equalizing  connection  is 
sometimes  used.  Series  motors  in  series  on  constant 
pressure  mains  will  operate  satisfactorily,  dividing  up  the 
total  voltage  between  them  according  to  the  load  each  is 
carrying.  If  it  be  desired  to  make  them  share  a  load 
equally  they  must  be  geared  together  so  that  each  rotates 
at  the  speed  corresponding  to  its  share  of  the  voltage. 
Series  motors  only  are  used  on  constant  current  circuits. 
Any  number  of  these  may  be  placed  in  series  on  such  a 


224  DYNAMO   ELECTRIC   MACHINERY. 

circuit  individually  or  connected  to  a  common  shaft.  A 
series  motor  on  a  constant  current  circuit  may  be  over- 
loaded until  it  stops  without  harm,  since  a  constant  current 
flows  at  any  speed. 

Compound-wound  motors  are  coming  into  quite  general 
use,  and  they  are  invariably  operated  on  constant  pressure 
circuits,  and  each  machine  has  its  own  load. 

In  ordinary  electric  railroad  practice,  as  has  been  stated, 
there  are  two  series-wound  motors  to  a  car,  operating 
either  in  series  or  parallel,  according  to  the  position  of  the 
controller  handle,  on  a  constant  pressure  of  500  volts. 
Each  of  these  motors  is  geared  to  a  separate  pair  of  driv- 
ing-wheels. Since  under  ordinary  conditions  the  rate  of 
rotation  of  the  two  motors  is  the  same,  the  E.M.F.  sup- 
plied to  each  is  the  same  when  they  are  in  series,  and 
since  the  current  is  common  they  divide  the  work  evenly. 
When  in  parallel  the  pressure  on  each  is  500  volts,  and 
since  the  rotations  are  the  same  the  currents  will  be  the 
same  and  the  load  will  be  divided  evenly.  It  often  occurs 
that  the  back  platform  of  a  car  is  so  loaded  that  the  front 
drivers  slip  when  the  power  is  applied  at  starting.  This 
occurs  when  the  motors  are  in  series  and  the  current 
is  common  to  the  two.  But  the  higher  rate  of  rotation  of 
the  front  motor  causes  it  to  generate  a  greater  counter 
E.M.F.,  thus  lowering  the  pressure  acting  on  the  rear 
motor.  Thus  more  electric  energy  is  consumed  in  the 
front  motor,  and  the  surplus  of  work  turns  into  heat  from 
the  friction  between  the  slipping  wheels  and  rails.  When 
the  car  gains  such  a  velocity  that  the  front  wheels  bite  the 
rails,  the  work  is  again  evenly  distributed  between  the  two 
motors.  It  should  be  remembered  that  this  occurs  only 
when  the  motors  are  in  series. 


MANAGEMENT   OF   MACHINES.  225 

116.  Care  and  Operation  of  Machines In  what  follows 

on  the  operation  of  motors  and  dynamos,  it  is  assumed 
that  the  machine  is  properly  designed  and  of  sufficient 
capacity  for  the  work  it  is  called  upon  to  perform.  For 
satisfactory  operation,  the  machine  must  be  connected  with 
an  appropriate  circuit  and  one  of  the  voltage  or  amperage 
for  which  the  machine  was  designed.  Further  it  is 
assumed  that  the  mere  mechanical  details  have  been  looked 
to,  such  as  proper  foundation,  proper  alignment  with  shaft- 
ing, and  good  lubrication.  Only  electrical  trouble  will  be 
treated. 

If  trouble  be  detected,  a  machine  should  be  at  once 
stopped  to  prevent  further  trouble.  In  central  generating 
stations,  one  of  the  most  positive  rules  is  not  to  shut 
down  while  any  possible  means  is  left  to  keep  running. 
In  such  plants  there  are  always  one  or  two  units  held  in 
reserve,  and  one  of  these  may  be  started  and  substituted 
for  a  machine  developing  a  fault  so  that  the  latter  may  be 
shut  down  and  its  fault  remedied. 

Sparking  at  the  brushes  is  the  most  general  trouble,  and 
it  has  more  causes  than  any  other.  The  brushes  must 
make  good  contact  with  the  commutator,  they  must  be 
true,  and  have  good  contact  surface.  The  commutator 
must  be  clean.  Any  collection  of  carbonized  oil  is  sure  to 
cause  sparking.  A  very  thin  layer  of  good  oil,  free  from 
dust,  is  advantageous.  On  a  bipolar  machine  the  brushes 
must  be  diametrically  opposite,  on  a  four  pole  exactly  90° 
apart,  etc.  This  condition  must  be  attained  while  the 
machine  is  at  rest,  either  by  actual  measurement  or  by 
counting  the  commutator  bars  between  each  brush.  If 
the  brushes  of  one  set  are  "staggered"  they  may  cover 
too  much  armature  circumference  and  cause  sparking. 


226  DYNAMO  ELECTRIC  MACHINERY. 

The  brushes  must  be  set  at  the  proper  point.  This  is 
accomplished  while  the  machine  is  in  operation  under  its 
required  load.  The  rocker  arm,  which  carries  all  the 
brush  holders,  is  moved  carefully  back  and  forth  until  the 
point  of  minimum  sparking  is  found.  Sometimes  there  is 
quite  an  arc  of  movement  in  which  sparking  is  not 
observed.  The  brushes  should  then  be  set  at  the  center 
of  this  arc,  since  heating  occurs  when  the  brushes  are  off 
the  proper  commutating  point,  even  if  sparks  be  not  seen. 

Sparking  may  be  due  to  fault  in  the  commutator.  A 
high-bar,  a  low-bar,  or  flat,  projecting  mica,  rough  or 
grooved  surface,  eccentricity,  or  any  condition  of  surface 
which  causes  the  brushes  to  vibrate  and  lose  contact  with 
the  commutator  will  surely  cause  sparking.  If  sparking 
be  allowed  to  go  without  correction,  it  will  pit  the  commu- 
tator and  aggravate  these  conditions.  If  the  irregularity 
of  surface  be  slight,  it  may  be  cut  down  by  sandpaper 
(never  emery)  held  in  a  block  cut  to  fit  the  commutator. 
If  the  surface  be  very  bad,  it  must  be  cut  down  by  a 
machine.  A  small  armature  may  be  swung  in  a  lathe  ;  but 
a  large  one  must  be  left  in  its  own  bearings,  and  a  tool 
held  against  the  commutator  by  some  special  device.  A 
perfectly  true  commutator  may  act  eccentrically  toward 
the  brushes  because  of  wear  in  the  shaft  bearings.  New 
bearings  will  remedy  this  fault. 

If  a  coil  of  the  armature  be  short-circuited,  periodic 
sparking  may  result.  The  coil  is  liable  to  burn  out  if  the 
machine  is  not  immediately  stopped.  The  short  circuit 
may  occur  from  breakdown  of  the  insulation  within  the 
armature,  in  which  case  rewinding  is  necessary ;  or  it  may 
be  caused  by  metal  chips  or  the  like  at  or  near  the  com- 
mutator, in  which  case  the  cause  can  be  easily  removed, 


MANAGEMENT   OF    MACHINES.  22/ 

When  a  coil  is  broken  very  violent  sparking  occurs,  since 
half  the  armature  current  is  broken  every  time  the  com- 
mutator bar  connected  to  the  broken  coil  passes  from  under 
a  brush.  Such  a  break  may  occur  within  the  armature, 
requiring  rewinding ;  but  it  is  more  likely  to  occur  where 
the  coil  end  is  attached  to  the  commutator  bar  lug.  If 
the  break  be  at  this  place,  the  wire  needs  but  to  be  screwed 
or  soldered  to  the  lug  and  the  machine  is  repaired. 

If  the  field  of  a  motor  is  too  weak,  sufficient  counter 
E.M.F.  is  not  generated,  and  excessive  current  flows  and 
causes  sparking.  The  weakening  of  the  fields  may  occur 
from  a  short  circuit  in  the  field  coils  or  two  or  more 
grounds  between  the  field  coils  and  the  pole  piece,  or  by  a 
broken  field  circuit  (shunt  coils)  which  reduces  the  mag- 
netism to  almost  zero.  In  any  case,  unless  the  trouble  is 
to  be  found  external  to  the  coils,  rewinding  is  necessary. 

Heating  of  machines  is  another  frequent  source  of 
trouble.  The  limit  of  temperature  that  may  be  allowed  in 
the  bearings  depends  on  the  flashing-point  of  the  lubricant 
used,  but  a  well  designed  and  lubricated  bearing  ought 
always  to  run  cooler  than  the  commutator  or  armature. 
The  limit  of  temperature  that  may  be  allowed  in  the  arma- 
ture depends  on  the  "  baking  "-point  of  insulation  used, 
and  also  on  the  melting-point  of  the  solder  used  if  the 
coil  ends  are  soldered  to  the  commutator  lugs.  A  good 
general  rule  is  this :  If  you  can  hold  your  hand  on  any 
part  of  the  machine  for  more  than  a  few  seconds,  that 
part  is  not  dangerously  hot.  Of  course  metal  feels  warmer 
than  insulating  cotton  for  the  same  temperature,  and 
allowance  should  be  made  for  this.  If  a  burning  smell  or 
smoke  comes  from  a  machine,  the  safe  temperature  limit 
has  been  far  exceeded,  and  the  machine  should  be  shut 


228  DYNAMO  ELECTRIC  MACHINERY. 

down  at  once.  This  indicates  a  serious  trouble  —  a  short 
circut  or  a  hot  bearing  probably. 

If  the  trouble  arises  from  the  bearings  the  ordinary  me- 
chanical precautions  of  cleaning,  aligning,  lubricating,  etc., 
will  generally  cure  it.  Never  use  water  to  cool  hot  bearings. 
If  water  gets  into  the  windings  of  either  the  field  or  the 
armature,  short  circuits  will  occur  and  ruin  the  machine. 
It  must  not  be  assumed  that  because  one  part  of  a 
machine  is  hot  the  trouble  lies  with  that  part.  Heat  is 
quickly  conducted  all  over  a  machine  ;  and  when  heat  is 
detected  in  one  place  the  machine  should  be  felt  all  over, 
the  hottest  part  probably  being  the  part  at  fault.  The 
brushes  of  a  machine  should  not  be  set  too  tight,  for,  be- 
sides reducing  the  efficiency  greatly,  they  cause  much 
heat  from  friction.  The  commutator  should  not  be  more 
than  5°  C.  hotter  than  the  armature. 

Machines  that  operate  on  constant  pressure  circuits  are 
liable  to  overheat  because  of  too  much  current  flowing 
through  some  parts  of  them.  This  may  result  from  over- 
loads, in  which  case  the  remedy  is  obvious,  or  because  of 
short  circuits  in  the  machines,  in  which  case  rewinding  is 
generally  necessary.  In  the  case  of  constant  potential 
generators  a  short  circuit  of  the  mains  will  produce  a  sud- 
den and  severe  overload,  which  can  only  be  remedied  by 
tracing  out  the  lines  and  removing  the  short  circuit. 

When  a  machine  makes  an  unwarranted  amount  of  noise 
it  usually  indicates  the  need  of  attention.  Carbon  brushes 
chatter  and  spark  sometimes  when  the  commutator  is 
sticky,  the  action  being  something  like  a  bow  on  a  violin 
string.  Cleaning  the  commutator  will  cure  this.  Hum- 
ming and  vibration  result  when  the  revolving  parts  are 
not  revolved  about  their  center  of  gravity.  This  may  be 


MANAGEMENT   OF   MACHINES.  229 

because  of  faulty  construction  or  warping  after  completion. 
If  the  fault  be  with  the  pulley,  it  may  be  turned  out  or 
counterweighted.  If  the  shaft  be  sprung  it  may  be 
straightened  or  a  new  one  used.  If  the  armature  core  or 
windings  be  out  of  balance,  there  is  not  much  help  for  it. 
Slower  speed  will  reduce  the  noise  from  this  cause. 

Noise  may  occur  from  the  armature  rubbing  or  striking 
against  the  pole  faces.  This  is  a  serious  matter,  and  if  not 
immediately  attended  to  results  in  the  destruction  of  the 
armature.  It  is  caused  generally  by  wear  in  the  shaft 
bearings,  in  which  case  new  brasses  will  remedy  the 
trouble.  Sometimes  it  results  from  a  sprung  shaft,  in 
which  case  the  shaft  must  be  either  straightened  or  re- 
placed. A  rattle  produced  by  loose  collars,  bolts,  nuts,  or 
connections  would  indicate  that  these  parts  needed  setting 
up  or  adjusting. 

If  a  motor  revolves  too  slowly,  it  may  be  because  of  an 
overload  of  mechanical  work,  because  of  excessive  friction 
in  the  machine,  or  because  of  the  armature  rubbing  against 
the  pole  face.  A  variation  in  the  pressure  supplied  to  a 
motor  causes  a  variation  in  speed.  If  the  field  magnetism 
be  too  weak  the  motor  will  revolve  too  fast  when  not 
loaded,  and  too  slow  when  under  full  load,  and  will  take 
excessive  current.  A  weak  field  may  be  caused  by  a  short 
circuit  which  cuts  out  some  or  all  of  the  field  turns,  or  by 
a  broken  field  circuit.  If  the  load  be  removed  from  a 
series  motor  on  a  constant  current  circuit  it  will  race  badly 
unless  its  field  coils  are  shunted.  Practically  such  a  motor 
should  not  be  used  in  any  position  where  it  may  be  sud- 
denly relieved  of  its  load,  as  by  the  slipping  of  a  belt.  A 
shunt  motor,  whose  fields  are  not  excited,  will  run  either 
forward  or  backward  when  a  current  is  allowed  to  flow  in 


230  DYNAMO   ELECTRIC   MACHINERY. 

the  armature,  according  to  the  relative  magnitudes  and 
directions  of  the  residual  magnetism  and  the  armature  re- 
actions. Ordinarily,  however,  if  a  motor  runs  backward,  it 
may  be  assumed  that  the  connections  are  wrong.  Usu- 
ally changing  the  connections  to  the  brush  holders,  so  that 
the  brushes  change  their  signs  without  changing  any  other 
connections,  will  make  the  motor  change  its  direction  of 
rotation.  A  series  motor  also  may  be  made  to  change  its 
direction  by  changing  the  direction  of  current  flow  in 
either  field  coils  or  armature,  but  not  in  both. 

On  starting  up,  a  self -exciting  dynamo  is  supposed  to 
build  up  its  voltage  to  normal,  having  at  first  no  excitation 
save  that  of  residual  magnetism.  After  standing  some 
time,  or  in  proximity  to  other  dynamos,  or  after  being 
hammered,  the  magnet  frame  may  have  lost  all  its  residual 
magnetism.  In  this  case  the  machine  does  not  build  up 
when  revolved.  By  passing  a  current  from  another 
machine  through  the  field  coils  the  dynamo  will  generate 
as  a  separately  excited  one.  Then  the  exciting  current 
may  be  thrown  off  and  the  self-excitation  thrown  on,  when 
the  machine  will  build  up  satisfactorily.  If  the  residual 
magnetism  becomes  changed  in  direction,  or  the  separately 
exciting  current  be  passed  in  the  wrong  direction,  then 
what  little  voltage  may  be  generated  will,  when  connected 
for  self-excitation,  send  the  current  in  such  a  direction  as 
to  tend  to  demagnetize  the  field,  and  building  up  will  be 
impossible.  A  shunt  machine  builds  up  better  the  less 
the  outside  load,  since  at  no  load  the  terminal  voltage  is  the 
greatest  and  the  most  likely  to  send  a  magnetizing  current 
in  the  field  coils.  A  series  machine  builds  up  better  when 
the  outside  load  is  increased.  Such  a  machine  may  even 
be  momentarily  short  circuited  to  make  it  build  up.  For  a 


MANAGEMENT   OF    MACHINES.  231 

given  voltage  resulting  from  residual  magnetism,  the  current 
in  the  field  coils  is  greater  the  less  the  resistance  in  the 
circuit.  If  the  connections  to  one  of  the  field  coils  in  a 
bipolar  machine  be  reversed,  causing  two  poles  of  the  same 
polarity,  the  machine  will  of  course  fail  to  generate.  This 
condition  may  be  detected  by  the  use  of  a  compass  needle. 
Small  machines  sometimes  generate  at  starting  a  few  volts, 
showing  proper  connections  and  the  presence  of  some 
residual  magnetism,  but  refuse  to  build  up  beyond  this 
point.  It  is  sometimes  convenient  to  materially  increase 
the  speed  of  such  a  machine,  whereupon  it  will  build  up 
rapidly,  and  the  speed  may  then  be  reduced  to  normal,  and 
the  dynamo  will  continue  to  generate  at  its  normal  pres- 
sure. 


232  DYNAMO   ELECTRIC   MACHINERY. 


CHAPTER   XIII. 

THE     DESIGN     OF     MACHINES. 

117.  Different  Methods  of  Design.  —  It  is  impossible  to 
lay  down  a  fixed  set  of  rules  to  be  followed  in  the  design 
of  dynamo  electrical  machinery.  This  is  because  the 
specified  conditions  of  operation  and  construction  are 
seldom  alike  in  two  cases.  A  designing  engineer  may  be 
called  upon  to  design  a  machine  of  a  given  output  at  a  given 
voltage,  the  field  frame,  however,  to  be  chosen  from  one  of  a 
set  already  in  stock  ;  and  again  it  may  be  required  that  the 
machine  shall  be  direct  connected,  the  output,  the  voltage, 
and  the  speed  of  rotation  being  given ;  still,  again,  the 
capacity,  the  maximum  gross  weight,  and  the  efficiencies  of 
operation  at  various  loads,  may  be  specified,  as  in  the  case 
of  an  automobile  motor  ;  or  he  may  be  called  upon  to  design 
a  machine  of  a  given  output  and  voltage,  which  shall 
operate  at  a  satisfactory  efficiency,  and  which  shall  have  a 
first  cost  which  will  enable  the  manufacturer  to  successfully 
compete  with  others  in  the  sale  of  his  products.  Through- 
out the  calculations  the  engineer  is  obliged  to  refer  to  his 
experience  or  the  experience  of  others  in  determining  the 
values  of  different  quantities  which  must  be  assumed  before 
there  can  be  any  further  progress  on  the  design.  Further- 
more, after  having  assumed  certain  values,  results  which 
are  arrived  at  later  on  in  the  work  will  necessitate  the  re- 
jection of  these  values  and  the  assumption  of  new  ones. 


THE   DESIGN   OF   MACHINES.  233 

Oftentimes  what  one  might  desire  as  a  value  for  one  quan- 
tity is  undesirable,  because  it  conflicts  with  the  adoption  of 
a  value  for  some  other  quantity  which  is  more  desirable.  In 
the  following  paragraphs  a  method  is  given  for  designing 
a  machine  under  the  conditions  specified. 

118.  Specifications. — The  following   specifications   are 
given  and  must  be  complied  with  : 

The  type  of  machine  as  regards  the  shape  of  its  field 
frame,  its  bearings,  and  the  method  of  its  being  driven  ;  its 
output  in  kilowatts ;  its  terminal  voltage  at  full  load  and  at 
no  load  ;  the  materials  from  which  are  to  be  constructed  its 
field  frame,  its  pole  pieces,  its  armature  core,  its  brushes, 
its  shaft,  its  bearings,  its  armature  spider,  and  its  con- 
ductors ;  and  the  insulation  throughout  its  various  parts. 

119.  Preliminary  Assumptions.  —  The   design   will   be 
based  upon  an  assumption  of  the  values  of  four  different 
quantities. 

The  first  assumption  is  that  of  the  value  of  the  flux  den- 
sity in  the  air  gap,  which  will  be  represented  by  ($>g.  The 
value  which  will  be  chosen  will  depend  somewhat  upon  the 
method  to  be  employed  for  obviating  armature  reaction. 
Almost  all  designers  rely  upon  a  stiff y  bristly  fie  Id  \.Q  assist 
in  preventing  a  distortion  of  the  field  when  under  load,  and 
therefore  higher  flux  densities  are  being  used  now  than 
were  a  few  years  ago.  Higher  densities  are  used  when  the 
pole  pieces  are  made  of  wrought  iron  or  of  cast  steel  than 
when  they  are  made  of  cast  iron.  The  densities  are  greater 
in  the  case  of  multipolar  machines  than  in  the  case  of 
bipolar ;  and  they  increase,  within  limits,  with  the  size  of 
the  machine.  A  value  between  4000  and  7500  should  be 
chosen. 


234  DYNAMO  ELECTRIC  MACHINERY. 

The  second  assumption  is  a  value  for  the  peripheral 
velocity  F'  of  the  armature  in  feet  per  minute.  The  com- 
mon assumption  in  the  case  of  drum  armatures  for  all  sizes 
above  five  K.W.  is  3000  feet  per  minute.  High-speed  ring 
armatures  have  a  higher  value,  ranging  between  4000  and 
6000.  The  larger  value  is  to  be  used  in  the  case  of  large 
machines. 

The  third  assumption  is  a  value  for  the  current  density 
in  the  armature  conductor  at  full  load.  Inspection  of  a 
large  number  of  machines  shows  the  use  in  many  of  them 
of  from  500  to  800  circular  mils  per  ampere.  Sometimes 
as  small  a  cross-section  as  200,  and  in  other  cases  as  large 
as  1 200  circular  mils  per  ampere,  have  been  found.  The 
low  value  is  used  in  the  case  of  machines  subjected  to 
periodic  loads  of  short  duration.  This  is  the  case  with 
elevator  motors,  pump  motors,  sewing-machine  motors, 
dental  drill  motors,  and  motors  on  special  machinery.  The 
high  value  is  used  on  machines  to  be  used  in  central  stations 
for  lighting  or  power  purposes.  The  specified  output  in 
kilowatts  divided  by  the  full-load  terminal  volts  gives  the 
total  current  output  of  the  machine  at  full  load.  This, 
divided  by  the  number  of  armature  circuits,  gives  the  cur- 
rent which  must  be  carried  by  each  conductor  at  full  load. 
This  current  multiplied  by  the  assumed  value  of  the  number 
of  circular  mils  per  ampere  gives  the  cross-section  of  the 
conductor  in  circular  mils.  Oftentimes  a  single  armature 
conductor  is  made  up  of  several  wires  in  multiple.  The 
multiplicity  of  wires  affords  pliability  in  winding,  and 
obviates,  to  a  certain  extent,  eddy  currents.  Again,  the  use 
of  copper  bars  for  windings  is  common,  they  being  in- 
sulated by  the  use  of  micanite,  fuller  board,  or  other 
sheet  insulating  materials.  A  cross-section  sketch  of  a 


THE   DESIGN   OF   MACHINES.  235 

single  conductor  should  be  made  in  which  the  dimensions 
are  given  of  the  copper  and  of  the  insulating  material. 

The  fourth  assumption  is  the  value  s  to  be  given  to  the 
polar  span,  s  represents  the  percentage  of  the  armature 
circumference  which  is  covered  by  the  faces  of  the  poles. 
This  value  varies  considerably  within  narrow  limits,  but 
unless  there  is  some  special  reason  for  the  assumption  of 
another  value  0.8  may  be  taken. 

120.    Design   of   the  Armature 1.    To  determine  the 

specific  induced  E.M.F.  in  volts  per  foot  of  active  con- 
ductor. yf 

E'  =  —j^>  x  jffi,  X  30.5  ^-8  volts. 

where  the  first  term  in  the  right-hand  member  represents 
the  velocity  of  the  moving  conductor  in  centimeters  per 
second,  the  second  term  represents  the  average  induction 
density  of  the  flux  which  enters  the  armature,  and  the 
third  term  consists  of  constants  to  reduce  feet  to  centi- 
meters, and  c.  g.  s.  units  to  volts. 

II.  To  obtain  the  length  of  active  conductor  I1  in  feet. 

I'  =  -=  X  number  of  armature  circuits. 

±L 

III.  To  obtain  the  number  of  active  conductors  S  upon 
the  armature. 

Let  ly  =  the  number  of  layers  in  the  armature  winding. 

p  =  the  assumed  ratio  of  the  length  to  the  diameter  of  the 

armature  core. 

d  =  the  mean  winding  diameter  of  the  armature  in  inches. 

w  =  the  specific  peripheral  width  of  one  armature  conductor 

in  inches.     (In  the  case  of  a  smooth-core  armature 

w  represents  the  width  of  the  armature  conductor 


236  DYNAMO   ELECTRIC   MACHINERY. 

plus  the  double  thickness  of  its  insulation,  both  in 
inches,  while  in  the  case  of  tooth  armatures  w  rep- 
resents the  width  of  one  tooth  plus  the  width  of 
one  slot  divided  by  the  number  of  conductors  in 
one  slot  in  one  layer.) 

The  length  of  the  armature  core  =  pd  inches  =  —  ft. 

and   the    circumference    of   the    core  =  ird   inches.      The 

_  vet 

IV 

"* 


number  of  armature  conductors  in  one  layer  =  —    hence 

w 


the  total  number  of  armature  conductors  >S  = 
Since 


In  practice  the  width  of  the  tooth  ranges  from  50  per 
cent  to  80  per  cent,  the  width  of  the  slot.  In  some  cases 
it  has  a  width  equal  to  that  of  the  slot.  The  value  for  5 
yielded  by  this  formula  must,  in  nearly  all  cases,  be  altered 
by  either  the  addition  or  subtraction  of  a  few  conductors 
in  order  to  make  it  possible  to  employ  the  type  of  winding 
which  it  seems  desirable  to  adopt.  The  change  may  neces- 
sitate a  slight  alteration  of  one  of  the  assumed  values,  and 
as  a  result  the  values  derived  from  it. 

For  machines  whose  speed  is  prescribed,  as  is  the  case 
with  direct  connected  machines,  one  may  use  the  form  of 


THE    DESIGN    OF    MACHINES.  237 

the  formula  5  =  — — ,  where  d  is  to  be  obtained  as  de- 

w 

scribed  in  the  end  of  the  next  paragraph. 

IV.  To  obtain  the  diameter  of  the  armattire  d  in  inches. 

Sw 

d  =  -2—  inches. 

7T 

In  case  the  speed  of  the  armature  in  revolutions  per 
minute  Fbe  prescribed,  as  is  the  case  with  direct  connected 
machines,  the  preliminary  assumption  of  the  peripheral 
velocity  V1  immediately  gives  a  value  for  the  armature 
diameter.  yt  ^,  ^ 

d  =  ——  ft.  =  — — —  inches. 
Vv  Vtr 

V.  To  determine  the  length  of  the  armature  I  in  inches. 


VI.  To  determine  the  internal  diameter  of  the  armature 
core  d'  in  inches.  In  determining  this  quantity  a  value  for 
the  flux  density  in  the  armature  core  (Ba  must  be  assumed. 
Wiener  states  that  in  incandescent  dynamos,  in  railway 
generators,  in  machines  for  power  transmission  and  distri- 
bution, and  in  stationary  and  railway  motors,  the  density 
varies  from  5,500  to  15,500.  Ring  armatures  have  higher 
densities  than  drum  armatures,  low-speed  machines  higher 
densities  than  high-speed  machines,  and  bipolar  machines 
have  larger  densities  than  multipolars.  (B0  =  8000  is  a 
good  assumption. 

If  the  machine  have  /  pairs  of  poles,  the  flux  which 
enters  the  armature  through  one  pole 


238  DYNAMO   ELECTRIC   MACHINERY. 

that  is,  the  surface  of  the  armature  in  square  centimeters 
times  the  average  gap  density  divided  by  the  number  of 
poles.  Considering  that  but  75  per  cent  to  80  per  cent  of 
the  length  of  the  armature  core  is  made  up  of  iron,  the 
rest  being  due  to  the  spaces  between  the  laminations  and 
the  width  of  the  ventilating  ducts,  the  radial  depth  of  the 
armature  core  is 


d-dr  = 


(2-54)2 


&a/0.75    (2.54)2 

VII.  To  determine  the  armature  losses.  The  armature 
as  already  determined  would  theoretically  operate  satis- 
factorily, but  there  is  a  possibility  of  its  heating  excessively 
when  running  under  full  load.  There  are  the  two  constant 
supplies  of  heat,  namely,  that  due  to  ohmic  resistance 
and  that  due  to  hysteresis  and  eddy  currents.  There  are 
also  two  avenues  for  the  escape  of  heat,  namely,  radia- 
tion and  air  convection.  An  equilibrium  is  established  when 
that  temperature  is  reached  which  will  make  the  escaping 
heat  per  unit  of  time  equal  to  the  amount  of  heat  gen- 
erated in  the  same  time.  Concerning  the  escape  of  heat 
by  radiation,  it  should  be  borne  in  mind  that  the  watts 
radiated  vary  as  the  difference  in  temperature  between  the 
radiating  body  and  the  surrounding  atmosphere  and  as  the 
emissivity  and  the  area  of  the  radiating  surface.  There 
is  also  on  starting  a  conduction  of  heat  to  neighboring 
bodies.  After  a  short  time,  however,  a  static  temperature 
condition  will  be  established.  The  power  loss  in  hysteresis 
in  the  armature  is 

i  V 

Ph  =  — _ -np ®>a*-r- v  watts. 

IO  DO 


THE    DESIGN    OF    MACHINES.  239 

where  ^  equals  the  hysteretic  constant  of  the  iron  (0.002),  v 
equals  the  volume  of  the  armature  core  in  cubic  centimeters. 
The  assumption  is  made  that  the  flux  density  in  the"  arma- 
ture core  is  uniform.  This  is  not  true  for  the  main  core,  as 
was  shown  by  Goldsborough,  and  in  the  teeth  the  density  is 
much  greater.  When  the  volume  of  the  latter  is  a  relatively 
large  amount  of  the  total  core  volume,  a  correction  should 
be  made.  When  making  many  designs,  in  which  the  same 
quality  of  iron  is  to  be  used,  it  is  much  easier  to  get  the 
hysteresis  loss  per  cubic  inch  at  various  densities  from 
tables  made  up  to  suit  the  iron.  The  power  loss  due  to 
ohmic  resistance 


=(—  " 

\number  of  ar 


— 
number  of  armature  circuits 

where  Imax  is  the  full-load  current  of  the  machine  in  am- 
peres, and  Ra  is  the  resistance  of  all  the  armature  con- 
ductors arranged  in  series.  Before  getting  Ph  and  Pr  one 
must  determine  the  values  in  VIII.  to  XL 

VIII.  To  obtain  the  armature  speed  V  in  revolutions  per 
minute.  This  quantity  is  prescribed  in  the  case  of  direct 
connected  machines.  In  other  cases  in  may  be  determined 
by  the  formula 


IX.     To  obtain  tJie  volume  of  the  armature  core  v  in  cubic 
centimeters. 


where  z  is  a  coefficient  which  represents  that  part  of  the 
armature  core  length  which  is  occupied  by  iron.  In  ordi- 
nary laminations  the  space  occupied  by  air  and  insulating 


240  DYNAMO   ELECTRIC   MACHINERY. 

oxide  on  the  plates  amounts  to  10  per  cent,  therefore  under 
these  circumstances  z  =  0.9.  The  introduction  of  ventilat- 
ing ducts  reduces  this  value  by  an  amount  which  can  be 
readily  determined. 

X.  To   obtain  the  resistance  of  the  armature   wire   in 
ohms.     The  total  length  of  the  armature  wire, 

/,  =  f  X  k, 

where  k  is  a  constant  greater  than  unity,  which  takes  into 
account  the  amount  of  dead  wire  employed  in  making  the 
end  connections.  This  value  depends  upon  the  value  of 
p  and  upon  the  method  of  winding.  In  the  case  of  formed 
coils  its  value  may  be  determined  from  measurements  upon 
a  single  coil.  This  value  is  generally  slightly  greater 
than  2.  Considering  that  the  resistance  of  a  hot  mil  foot  is 
11.5  ohms,  the  resistance  of  the  armature 

£  11.5   I'* 

cross-section  in  circular  mils. 

XI.  To  obtain  the  area  of  the  armature  radiating  surface 
A  in  square  inches, 


XII.    As  2  to  2\  watts  can  be  radiated  per  square  inch 
of  armature  surface  without  excessive  heating,  the  value  of 
p  _j_  p 
— r  determines  whether  the  armature  is  properly  de- 

yi 

signed  or  not.  If  the  fraction  is  less  than  2,  the  armature 
is  needlessly  large,  and  should  be  redesigned.  If  the  frac- 
tion is  greater  than  2\,  the  armature  will  heat  excessively, 
and  should  also  be  redesigned. 


THE    DESIGN    OF    MACHINES.  241 

121.  Design  of  the  Field.  —  XIII.  Dimensions  of  the 
poles  and  field  frame.  The  design  of  a  field  requires  judg- 
ment and  experience  on  the  part  of  the  designing  engineer, 
and  an  acquaintance  with  the  various  machines  of  the  type 
being  designed.  One  must  assume  values  for  the  following 
quantities :  the  flux  density  in  the  poles  fl^,  the  flux 
density  in  the  magnet  frame  %  the  coefficient  of  magnetic 
leakage  A,  and  the  ratio  of  the  length  of  a  pole  to  its  diam- 
eter in  case  it  has  a  circular  cross-section,  or  to  some  other 
dimension  in  case  it  is  not  circular.  The  assumption  is 
made  here  that  the  field  frame  is  of  a  circular  type,  and 
that  the  pole  is  of  circular  cross-section.  It  is  customary 
to  choose  such  a  value  for  (&p  that  the  magnetization  will 
be  carried  over  the  knee  of  the  magnetization  curve.  In 
the  case  of  (By-,  however,  it  is  customary  to  choose  a  value 
somewhat  below  the  knee.  The  coefficient  of  magnetic 
leakage  for  this  type  of  machine  is  1.4.  A  careful  design 
really  requires  a  knowledge  of  the  distribution  of  the  leak- 
age flux.  Long  experience  enables  one  to  make  allow- 
ance for  this.  From  these  assumed  values  one  gets  a 
value  for  the  cross-section  of  a  pole, 

<M 
A   =  — —  sq.  centimeters, 

<Bp 

whence  it  follows  that  the  diameter  of  the  pole  in  inches 
dp  =  2.54  V/ ^  inches,  and  the  cross-section  of  the  frame, 

T  7T 

<M 
Af  =  -?-—  sq.  centimeters. 

2  (jjf 

XIV.  Reluctance  of  the  magnetic  circuit.  After  mak- 
ing a  provisional  scale-drawing  of  the  field-magnet  frame 
with  its  poles  and  the  armature  core,  exercising  judgment 
derived  from  experience  or  from  the  inspection  of  other 


242 


DYNAMO   ELECTRIC   MACHINERY. 


drawings,  determine  the  average  length  in  centimeters  of 
the  path  of  the  magnetic  lines  in  the  frame,  in  the  poles, 
in  the  air  gap,  in  the  teeth,  and  in  the  armature  core. 


Fig.    176. 

Represent  by  lf,  lp,  lg,  lt  and  lt  the  length  in  centimeters 
of  the  parts  marked  in  Fig.  176.  From  the  assumed 
values  of  the  flux  density,  and  from  the  magnetization 
curves  of  the  metals  from  which  the  various  parts  of  the 
magnetic  circuit  are  constructed,  one  can  get  the  respec- 
tive permeabilities.  The  reluctance  may  then  be  calculated 
as  follows  : 

Reluctance  of  the  pole  (Rp  = 


of  4-  section  of  field  frame  (R/-  =  —  —-  . 

/ 


of  the  air  gap  (R  = 


of  \  section  of  the  armature  core, 


THE   DESIGN   OF   MACHINES. 


243 


To  determine  the  reluctance  offered  by  the  teeth  and 
winding-slots,  it  is  convenient  to  assume  that  the  total  flux 
is  carried  by  the  teeth  alone.  Owing  to  the  fringing  of 
the  field  at  the  pole  tips,  not  merely  the  teeth  immediately 
under  the  pole  face  carry  the  flux  from  that  pole,  but,  with 
very  short  air  gaps,  an  extra  tooth  takes  part  in  the  trans- 

TABLE    OF   TOOTH-DENSITY   CORRECTIONS. 


CORRECTED  IRON 

DENSITIES  ON  THE  ASSUMPTION  THAT  THE  IRON  TRANSMITS 

DENSITY. 

THE  ENTIRE  FLUX. 

LINES  PER  SQ.  CEN- 

TOOTH WIDTH  = 

TOOTH  WIDTH  = 

TOOTH  WIDTH  = 

TIMETER. 

SLOT  WIDTH. 

|  SLOT  WIDTH. 

i  SLOT  WIDTH. 

17050 

17200 

17380 

17510 

18000 

18450 

18600 

18800 

19050 

19680 

2OOOO 

2O2OO 

2OOOO 

21050 

2I3OO 

21850 

2I02O 

22200 

23OOO 

23700 

22OOO 

24000 

24800 

25500 

23100 

26OOO 

26800 

28400 

1 

TOOTH  DENSITY  CORRECTION  CURVES 


§23 

2 

!22 
I* 

I20 

z 
o  19 

jr  18 

T-WI 
S  =WI 

)TH  OF 
)TH  OF 

TOOTH 
SLOT 

^H 

4=-75 

^ 

S^ 

-S-* 

/, 

<^ 

S^- 

A 

^ 

> 

V 

'/ 

/6 

^ 

^ 

^ 

17         18  VJ         20          21  22         23          M.        tt          !M 

UNCORRECTED  DENSITY'   KILOLINE8  PER  SQ.  CM. 

Fig.  177. 


244 


DYNAMO   ELECTRIC   MACHINERY. 


mission.  With  large  air  gaps  two  or  three  extra  teeth  may 
take  part.  The  value  of  the  permeability  obtained  from 
the  flux  density  which  is  calculated  upon  the  above  as- 
sumption would  be  too  small.  The  value  of  the  reluc- 
tance based  upon  it  would  in  consequence  be  too  large. 
The  flux  density  arrived  at  will  have  to  be  corrected  by 
reference  to  the  table  on  page  243. 

The  permeability,  ^tt  corresponding  to  the  corrected  dens- 


23000 


21000 


SB 
20000 


19000 


18000 


17000 

100   200  300  400  500  600   700  800  900  1000  1100  1200  1300  1400  1500 
10   20   30   40   50   60    70   80   90   100  110  120  130  .140  150  '160  [i 

Fig.  178. 

ity  and  to  be  obtained  from  Fig.  178,  should  be  inserted  in 
the  formula  for  the  reluctance  of  the  teeth, 


THE   DESIGN   OF    MACHINES.  245 

where  At  is  the  net  iron  cross-section  of  the  teeth  under 
one  pole  corrected  for  fringing.  The  reluctance,  the  flux 
through  which  must  be  maintained  by  the  field-windings 
on  one  pole,  is  made  up  of  a  bi-parallel  path  in  the  arma- 
ture and  a  bi-parallel  path  in  the  field  frame,  both  arranged 
in  series  with  the  pole,  the  gap,  and  the  tooth  reluctances. 
This  reluctance  is  equal  to 


XV.  Magneto-motive  force.  The  ampere  turns  per 
pole  nl^  necessary  to  produce  the  flux  <f>a  in  the  armature 
at  no  load  is  equal  to 


These  ampere  turns  are  furnished  by  the  shunt  coil  on 
one  pole. 

XVI.    SJiunt  Coils.     Assuming  that  Eb  volts  are  con- 
sumed in  the  field  regulating  rheostat, 

n  E  —  Eh  '        IA 

-"«*= 


circular  mils' 
Whence, 

The  cross-section  in  circular  mils  =  II'5//  «*  ^2/>t 

E  —  Eb 
Where  n  —  number  of  turns  in  shunt  coil, 

7sA  =  the  current  in  the  shunt  at  no  load,  and 
!sh  =  the  mean  length  of  one  field  turn  in  feet. 

Assuming  1,000  circular  mils  per  ampere  in  the  shunt  coil, 

_       circular  mils 

,  and 


n  = 


1000 

1000  n 


circular  mils 


246  DYNAMO   ELECTRIC   MACHINERY. 

From  a  wire  table  the  space  occupied  by  the  n  turns  can 
be  attained ;  and,  with  due  allowance  for  insulation,  refer- 
ence to  the  preliminary  drawing  will  enable  one  to  deter- 
mine whether  the  assumed  length  of  the  pole  lp  is  too 
small  or  too  great.  Space  must  be  left  for  the  compound 
coil.  This  occupies  about  one-half  as  much  space  as  the 
shunt  coil.  If  lp  seems  of  unsuitable  length  it  should  be 
altered,  and  the  calculation  should  be  again  gone  over. 

XVII.  Compound  Coils.  The  method  of  calculating 
the  number  of  compounding  turns  is  so  similar  to  that  in 
the  case  of  shunt  coils  that  it  need  not  be  gone  into  in 
detail.  The  compound  coils  have  to  compensate  at  full 
load  for  drop  in  the  armature,  for  drop  in  the  series  coil, 
for  drop  in  the  line  in  case  of  overcompounding,  for  the 
demagnetizing  armature  ampere  turns,  and  for  changes  in 
reluctance  due  to  skew  by  saturation.  The  back  armature 
ampere  turns,  when  multiplied  by  the  coefficient  of  mag- 
netic leakage,  give  the  series  ampere  turns  necessary  to 
compensate  for  them.  It  should  be  borne  in  mind  that 
the  maximum  possible  lead  brings  the  brush  no  farther 
than  the  pole  tip.  To  compensate  for  a  drop  of  a  certain 
percentage  requires  that  the  density  in  the  air  gap  be 
raised  by  that  same  percentage.  This  necessitates  an 
increase  of  all  the  densities.  The  increase  of  each  reluc- 
tance and  the  increase  of  each  corresponding  flux  must  be 
cared  for  by  the  series  windings.  The  coefficient  of  mag- 
netic leakage  varies  with  the  load.  The  manner  of  its 
variation  may  be  unknown.  The  reluctances,  into  which 
it  enters,  are  such  a  small  per  cent  of  the  total,  that  its 
variation  may  often  be  neglected. 

The  following  blank  form,  to  be  filled  in  by  students  in 
designing,  is  self-explanatory. 


THE   DESIGN   OF    MACHINES. 


247 


POLYTECHNIC    INSTITUTE   OF   BROOKLYN. 

DEPARTMENT   OF    ELECTRICAL    ENGINEERING. 

Data  sheet  to  be  filled  in  by  students  taking  Electrical  Engineering  8,  and 
covering  the  work  of  the  first  semester.  This  must  be  accompanied  by  the 
following  scale  drawings :  end  elevation,  longitudinal  cross-section,  plan,  and 
important  details.  A  diagram  of  the  armature-winding  must  also  be  given. 
Assumed  values  are  to  be  entered  in  red  ink. 


....Designer  Submitted 


SPECIFICATIONS. 

1.  Type  of  Machine 

2.  Number  of  poles 

3.  Capacity  in  kilowatts 

4.  Terminal  volts  at  no  load 

5.  Terminal  volts  at  full  load 

6.  Amperes  at  full  load 

7.  Revolutions  per  minute 

MATERIALS. 

8.  Armature  core 

9.  Armature  spider 

10.  Armature  end  plates 

n.  Armature  shaft 

12.  Commutator  segments 

13.  Commutator  spider 

14.  Magnet  frame 

15.  Pole  piece 

1 6.  Pole  shoe 

17.  Brushes 

1 8.  Brush-holders 

19.  Brush-holder  yoke 

20.  Commutator  insulation 

21.  Armature  conductor  insulation 

22.  Field-coil  insulation 

DIMENSIONS. 
Armature. 

23.  Diameter  over  all 

24.  Diameter  at  bottom  of  slots 


25.  Internal  diameter  of  core 

26.  Length  over  conductors 

27.  Length  of  core  over  laminations 

and  ducts 

28.  Net  length  of  iron 

29.  Number  of  ventilating  ducts 

30.  Width  of  each  ventilating-duct 

31.  Thickness  of  sheets 

32.  Number  of  slots 

33.  Depth  of  slots 

34.  Width  of  slot  at  root 

35.  Width  of  slot  at  surface 

36.  Width  of  tooth  at  root 

37.  Width    of    tooth   at   armature 

face 

38.  Size   and   shape   of  bare   con- 

ductor 

39.  Size  of  conductor  insulated 

40.  Pitch  of  winding,  No.  of  teeth 

41.  Arrangement  of  wires  or  bars 

per  slot 

42.  Number  in  parallel  per  slot 

43.  Number  in  series  per  slot 

44.  Total   insulation   between   con- 

ductors 

45.  Thickness    insulation    between 

conductors 

Air  Gap. 

46.  Length  in  center 


248 


DYNAMO  ELECTRIC  MACHINERY. 


47.  Length  maximum 

48.  Bore  of  field 

49.  Minimum  clearance 

Pole  Shoe. 

50.  Length  parallel  to  shaft 

51.  Length  of  maximum  arc 

52.  Length  of  minimum  arc 

53.  Minimum  thickness 

Poles. 

54.  Length  of  pole 

55.  Width  or  diameter  of  pole 

56.  Lenpth  parallel  to  shaft 

Magnet  Spool. 

57.  Number  of  spools 

58.  Length  over  all 

59.  Length  of  winding-space 

60.  Depth  of  winding-space 

Magnet  Frame. 

61.  External  diameter 

62.  Internal  diameter 

63.  Thickness 

64.  Diameter  over  ribs 

65.  Thickness  of  ribs 

66.  Length  along  armature 

Commutator. 

67.  Diameter 

68.  Number  of  segments 

69.  Width  of  segment  at   commu 

tat  or  face 

70.  Width  of  segment  at  root 

71.  Useful  depth  of  segment 

72.  Thickness  of  mica  insulation 

73.  Available  length  of  surface  of 

segments 

74.  Total  length  of  commutator 

75.  Peripheral  speed 

Brushes. 

76.  Number  of  sets  of  brushes 

77.  Number  in  one  set 


78.  Length 

79.  Width 

80.  Thickness 

81.  Area  of  contact  one  brush 

ELECTRICAL. 
Arn  ature. 

82.  Voltage  at  no  load 

83.  Total  voltage  at  full  load 

84.  Total  current 

85.  Number  of  sections 

86.  Turns  per  section 

87.  Number  of  layers 

88.  Total  number  of  inductors 

89.  Type  of  winding  .  .  .  circuits 

90.  Style  of  winding 

91.  Circular  mils  per  ampere 

92.  Mean  length  of  one  turn 

93.  Total  length  of  arm  wire 

94.  Resistance  of  armature  cold  at 

20°  C.  .  .  .  ohms 

95.  Resistance  of  armature  hot  at 

70°  C.  .  .  .  ohms 

Shunt  Coils. 
96     Size  of  wire,  No.  B.  &  S.  Gauge 

97.  Turns  per  layer 

98.  No.  of  layers 

99.  Turns  per  spool 

100.  Mean  length  of  one  turn 

101.  Total  turns 

102.  Total  length  of  wire 

103.  Total  weight  of  wire 

104.  Total  lesistance  at  .  .   .  20°  C. 

ohms 

105.  Total  resistance  at  ...  70°  C. 

.  .  .  ohms. 

1 06.  Volts  allowed  for  rheostat 

107.  Maximum   current    .    .    .    am- 

peres 

1 08.  Total  ampere  turns 

109.  Circular  mils  per  ampere 


THE   DESIGN   OF   MACHINES. 


249 


Series  Coils. 

1 10.    Size  and  shape  of  conductor 
in.    Number  of  conductors  in  mul 
tiple 

112.  Arrangement 

1 13.  Turns  per  layer 

114.  Number  of  layers 

1 1 5.  Turns  per  spool 

1 1 6.  Mean  length  of  one  turn 

117.  Total  turns 

1 1 8.  Total  length  of  conductor 

119.  Total  resistance  at  .  .  .  20°  C. 

.  .  .  ohms. 

1 20.  Total  resistance  at  .  .  .  70°  C. 

.  .  ,  ohms. 

121.  Maximum    current    .    .    .    am- 

peres 

122.  Total  ampere  turns 

123.  Circular  mils  per  ampere 

HEATING. 
Armature. 

124.  Area    of   drum    radiating  sur- 

face .  .   .  sq.  in. 

125.  Area  each   end   radiating  sur- 

face .  .  .  sq.  in. 

126.  Total    radiating    surface   .  .    . 

sq.  in. 

127.  /'/'full  load  .  .  .  watts 

128.  Hysteresis  .  .  .  watts 

129.  Eddy  currents  .  .  .  watts 

130.  Total  .  .  .  watts 

131.  Total    J"R   and   core   loss   at 

full  load  .  .  .  watts 

132.  Watts  per  sq.  in.  radiating  sur- 

face, full  load 


133.  Estimated  rise  of  temperature 

at  full  load  .  .  .  C. 

134.  Friction  of  windage  and  bear- 

ings .  .  .  watts 

Field  Coils. 

135.  Radiating  surface  (heads)  .  .  . 

sq.  in. 

136.  Radiating  surface  (periphery) 

.  .  .  sq.  in. 

137.  Total   radiating   surface  .  .   . 

sq.  in. 

138.  1*R  shunt  coils  and   rheostat 

.  .  .  watts 

139.  I*R  series  .  .  .  watts 

140.  Total    J*R  .   .  .  watts  .  .  .  % 

141.  Watts  loss  per  sq.  in.  .  .  .  radi- 

ating surface 

142.  Estimated  rise  of  temperature 

at  full  load      .  .  C. 

Commutator, 

143.  Brush  friction  .  .  .  watts 

144.  Brush  contact   .  .  ,  watts 

145.  Other  commutator  losses  .  . 

watts 

MAGNETIC. 

146.  fyi  at  open  circuit 

147.  fa  a<  full  load 

148.  Leakage  coefficient 

149.  <fcf  at  open  circuit  .  .  ,  <£/"  at 

full  load 

150.  Ampere    turns     required     for 

shunt 

151.  Ampere     turns     required     for 

series 


250    RELUCTANCES   AND   AMPERE   TURNS    PER   POLE. 


H5| 


dVO'l 

ON 


O'tf 

£3 

fa    CJ 


s 
^ 


•o  '2   *   6 

I  >  IS 


tfi  S 

.     H  CJ  W 

^  CO      Tf      tr> 

U       \O    ^O    MD 


^2    i*    c 
^  .S  3 


-     0    ^ 


•a  "3  ts 

Hs 

Q\    O      M 
io  \O    vo 


TESTS. 


251 


CHAPTER   XIV. 

TESTS. 

122.  Determination  of  a  (B-OC  Curve. — The  most  exact 
laboratory  method  of  rinding  this  curve  is  by  the  ballistic 
galvanometer  or  ring  method.  Fig.  179  shows  the  arrange- 
ment of  apparatus  for  this 
method.  X  is  the  test  piece  in 
the  form  of  an  annular  ring,  hav- 
ing a  mean  circumference  of  / 
centimeters  and  a  radial  cross- 
section  of  a  sq.  centimeters.  It 
is  wound  uniformly  with  n  prim- 
ary turns  of  wire.  Over  these 
three  or  four  secondary  test  turns 
of  wire  lead  off  to  the  ballistic 
galvanometer  G.  A  series  cir- 
cuit is  formed  of  a  storage  bat- 
tery or  other  suitable  source  of 

E.  M.  F.,  B,  a  variable  resistance  R,  the  primary  coil  of 
the  test  piece,  an  ammeter  A,  and  a  com  mutating  switch  C. 
The  last  is  used  for  reversing  the  direction  of  the  current 
in  the  primary  coil. 

If  R  be  adjusted  to  give  a  current  7,  then  the  magne- 
tizing force,  or  5C,  by  §  2 1  is  represented  by  the  formula 


Fig.  179. 


252 


DYNAMO   ELECTRIC   MACHINERY. 


If  now  the  current  be  suddenly  commutated,  all  the  lines  of 
force  will  be  withdrawn,  and  as  many  more  set  up  in  the 
opposite  direction.  Each  of  these  lines  will  induce,  upon 
commutation,  a  pressure  in  each  turn  of  the  test  coil. 
This  induced  E.M.F.  furnishes  a  means  of  measuring  the 
flux  of  lines  in  the  test  piece  or  the  flux  density  (B.  By 
application  of  the  formula  given  in  §  16,  one  may  obtain 
the  expression  for  this  quantity, 


Where 

J?t=  the  resistance  of  the  test  coil,  the  galvanometer,  and 

the  secondary  circuit ; 
a  =  the  area  of  a   radial    section   of   the   test   piece    in 

square  centimeters ; 

»2=  the  number  of  turns  in  the  test  coil ; 
k  =  a  constant  of  the  galvanometer;  and 
0  =  the  throw  of  the  galvanometer  which  accompanies  the 

commutation  of  the  primary  current. 

Though  the  most    accurate  method,  the  ring  method  is 
not  generally  employed  in  commercial  practice  because  of 

the  cost  and  the  time  re- 
quired in  preparing  a  test 
piece. 

The  divided-bar  method 
admits  of  the  use  of  a  bar 
of  iron  of  ordinary  shape  as 


TEST 
COIL 

V  TO  V--MEAN_CENGTH 
L  OF  TEST  PIECE 


l8°- 


into  two  pieces.  A  heavy 
wrought-iron  yoke,  Fig.  180,  has  a  magnetizing  coil  wound 
inside  of  it.  Through  snug-fitting  holes  in  the  ends  of 
the  yoke,  the  two  halves  of  the  test  piece  are  inserted, 


TESTS. 


253 


one  being  secured,  and  the  other  being  fitted  with  a 
handle.  The  test  coil  is  so  mounted  on  springs  as  to 
fly  suddenly  to  one  side  when  the  test  pieces  are  slightly 
separated  by  a  pull  on  the  handle.  It  thus  cuts  all 
the  flux  in  the  piece,  and  affords  a  means  of  measuring 
it.  The  yoke  is  so  massive,  and  has  such  a  small  reluc- 
tance as  compared  with  that  of  the  test  piece,  that  the 


formula  3C  =  is  practically  true,  where  /  is  the  mean 

10  / 

length  of  the  test  piece  which  is  traversed  by  magnetic 
lines.  For  (B  the  formula  is  twice  what  it  was  in  the 
ring  method,  since  the  test  coil  cuts  the  flux  but  once,  or 


tkO. 


The  method  of  reversals  could  be  used  equally  well  with 
this  apparatus,  requiring  the  formula  used  in  the  ring 
method. 

The  permeameter  is  a  machine  for  measuring  the  flux  in 
a  test  piece  by  measuring  the  force  necessary  to  detach  it 
from  another  part  of  the  magnetic  cir- 
cuit. Fig.  1 8 1  shows  in  simple  such  a 
machine.  The  magnetizing  force  is 
supplied  by  the  coil  C  the  same  as  in 
the  divided  bar  method.  The  test  coil 
and  galvanometer  are  done  away  with. 
The  bottom  of  the  yoke  Y  is  surfaced 
to  receive  flatly  the  end  of  the  test  rod 
T.  When  the  proper  current  is  flow- 

r  Fig.  x8x. 

ing  in  the  coil,  the  force  necessary  to 

separate  the  test  piece  from  the  yoke  is  found  by  means  of 

the  spring-balance  S.     Since  the  force  required  to  break 


254  DYNAMO  ELECTRIC    MACHINERY. 

any  number  of  lines  of  force  varies  as  the  square  of  that 
number,  it  is  easy  to  calculate  the  flux,  and  since 

<£  =  area  X  ®, 

the  magnetic  density  is  readily  found.  The  value  of  3C  is 
obtained  as  in  the  preceding  case. 

123.   Determination   of    the  Ballistic    Constant.  —  The 

standard  condenser  affords  the  most  convenient  and  accu- 
rate means  of  determining  the  constant  of  a  ballistic  gal- 
vanometer. If  the  capacity  of  the  condenser  C,  and  the 
voltage  at  which  it  is  charged  E,  be  known,  then  the  quan- 
tity of  electricity  that  will  flow  when  the  circuit  is  closed 
through  the  galvanometer  is  also  known.  It  is  equal  to 
EC.  By  observing  the  galvanometer  throw  0,  the  value  of 
the  constant  k  is  determined  from 


The  coil  of  a  d'  Arsonval  ballistic  galvanometer  moving  in 
its  field  has  an  E.M.F.  induced  in  it,  which  tends  to  send 
a  current  in  a  direction  opposite  to  that  of  the  current  that 
produces  the  throw,  and  which,  therefore,  shortens  the 
throw  or  damps  the  galvanometer.  The  magnitude  of  this 
damping  current  depends,  of  course,  on  the  resistance  of 
the  galvanometer  circuit  ;  hence  the  constant  k  should  be 
determined  with  the  same  external  resistance  in  the  gal- 
vanometer circuit  as  there  will  be  when  the  test  for  the 
value  of  &  is  being  made. 

To  accomplish  this  an  arrangement  of  apparatus  such  as 
is  shown  in  Fig.  182,  may  be  employed,  the  particular  fea- 
ture of  which  is  the  quadruple  contact  key.  This  key  is 


TESTS. 


255 


normally  held  up  against  a  contact.  In  this  position  the 
galvanometer  circuit  is  open,  and  the  condenser  is  in  series 
with  a  charging  battery. 
As  the  key  is  pressed  down, 
three  things  occur.  First 
the  battery  circuit  is  broken, 
then  the  condenser  is  dis- 
charged through  the  gal- 
vanometer, and  lastly  the 
galvanometer  circuit  is 
closed  through  an  appro- 
priate amount  of  resistance 
in  the  rheostat. 

The  ballistic  constant  may  also  be  determined  by  the 
use  of  a  long  solenoid,  with  a  few  turns  about  its  center 
for  a  test  coil.  A  series  circuit  is  formed  (Fig.  183)  of 
a  battery,  a  variable  resistance,  an  ammeter,  the  solenoid, 


fomrnmsm 

^— 


Fig.  183. 


and  a  key.  The  ends  of  the  test  coil  are  attached  to  the 
galvanometer  through  proper  resistance.  On  closing  the 
circuit  the  current  /  sets  up  a  field  at  the  center  of  the 
solenoid,  whose  intensity, 


256  DYNAMO   ELECTRIC   MACHINERY. 

where  /  is  the  length  of  the  solenoid  in  centimeters,  and  n 
the  number  of  primary  turns. 

If  A  be  the  area  in  square  centimeters  of  a  cross-section 
of  the  solenoid  perpendicular  to  the  axis,  then 


If  n2  be  the  number  of  turns  in  the  test  coil,  and  R  be 
the  resistance  of  the  galvanometer  circuit  in  ohms,  then 
upon  closing  the  circuit  and  upon  establishing  the  flux  <f>,  a 
quantity  of  electricity  will  pass  around  the  secondary  cir- 
cuit which  is  equal  to 

n  -  w  -  n^ 

~8 


where  0  is  the  throw  of  the  galvanometer  corresponding  to 
the  current  /.     Therefore, 


If  the  solenoid  be  less  than  ten  diameters  long,  this  result 
is  not  accurate,  owing  to  the  influence  of  the  ends  of  the 
solenoid  upon  the  value  of  JC. 

There  have  been  described  numerous  methods  for  deter- 
mining k  which  depend  upon  a  constant  and  definite  inten- 
sity of  the  earth's  magnetic  field.  Nowadays  the  fact 
that  the  earth's  field  is  constantly  changing,  both  in  direc- 
tion and  magnitude,  due  to  the  prevalence  of  iron  and  steel 
buildings,  and  the  extensive  use  of  electric  currents  for 
trolley,  lighting,  and  other  purposes,  makes  these  methods 
practically  worthless. 

124.   Determination  of   the  Hysteretic  Constant.  —  The 

hysteresis  curve  for  any  sample  of  iron  may  be  found  most 
accurately  by  the  step-by-step  method.  The  arrangement 
of  apparatus  is  in  all  respects  similar  to  that  of  Fig.  179, 


TESTS.  257 

save  that  the  rheostat  R  must  be  so  designed  that  the  cir- 
cuit does  not  open,  even  for  an  instant,  in  passing  from 
one  resistance  point  to  another.  The  method  of  operation 
is  as  follows  :  The  rheostat  is  set  for  a  maximum  current 
strength  which  is  determined  by  means  of  an  ammeter. 
The  rheostat  handle  is  then  quickly  moved  back  one  point. 
This  reduces  the  current  and  the  dependent  magnetizing 
force  proportionately.  There  is  an  accompanying  decrease 
of  flux  in  the  sample.  This  decrease  is  determined  by  the 
galvanometer  throw,  the  formula  being  as  before, 

Change  in  <&  =  ^-^- kO. 
an^ 

The  ammeter  current  is  again  read ;  and,  as  soon  as  the 
galvanometer  comes  to  rest,  the  resistance  is  increased  by 
another  step,  and  the  throw  of  the  galvanometer  is  observed. 
After  the  current  has  been  reduced  step  by  step  to  zero,  it 
is  then  commutated,  and  increased  by  steps  until  the  maxi- 
mum magnetization  is  obtained  in  a  direction  opposite  to 
that  at  the  beginning.  The  current  is  again  cut  down  by 
steps  to  zero,  is  afterwards  commutated  for  a  second  time, 
and  is  again  increased  until  the  magnetic  condition  of  the 
iron  which  prevailed  at  the  start  is  again  attained.  Giving 
to  (B  a  plus  or  minus  sign,  according  to  the  direction  of  the 
galvanometer  throw,  the  algebraic  sum  of  all  the  changes 
of  (B  must  equal  zero.  Therefore  the  algebraic  sum  of 
all  the  galvanometer  throws  should  equal  zero.  A  simple 
addition  serves  as  a  check  on  all  observations.  In  practice, 
the  sum  of  the  plus  throws  may  differ  from  the  sum  of  the 
minus  ones  by  three  per  cent  without  seriously  affecting  the 
final  result.  Having  the  maximum  value  of  (B,  the  (B's  cor- 
responding to  each  can  readily  be  found  by  subtracting  the 


258 


DYNAMO   ELECTRIC   MACHINERY. 


changes  in  (fc  from  the  maximum  <B.  Upon  plotting  a  cyclic 
curve  of  the  various  values  of  (fc  and  the  corresponding 
values  of  5C,  one  obtains  a  hysteresis  loop,  as  in  Fig.  184. 
The  area  of  this  loop  in  (BX  units,  when  divided  by  4?r,  gives 
the  ergs  loss  of  energy  in  carrying  one  cubic  centimeter  of 
the  iron  under  test  through  a  cycle  of  magnetization  be- 
tween the  limits  of  +  ®>max  and  —  ($>max.  According  to  the 
Steinmetz  formula, 

h  =  rj^max, 

where  h  is  the  loss  by  hysteresis  in  ergs  per  cycle  per  cubic 
centimeter.  Hence,  to  find  the  hysteresis  constant  17  of 
the  sample  used  in  the  foregoing  test,  one  uses  the  formula 


where    Ah=.  area  of  hysteresis  curve  expressed  in  (B3C  units, 
and          V '=  volume  of  iron  in  cubic  centimeters. 


U  ague  fixing 


Fig.    184. 


TESTS. 


259 


A  much  less  laborious  method  of  measuring  rj,  and  one 
which  does  not  introduce  the  errors  attending  the  measure- 
ment of  the  area  of  a  curve,  is  the  wattmeter  method.  Since 
the  iron  to  be  tested  is  generally  for  use  in  alternating  cur- 
rent apparatus,  this  method  has  the  additional  advantage 
that  the  test  occurs  under  the  conditions  which  the  iron 
will  meet  in  its  working. 

If  the  ring  be  made  of  annular  stampings  of  sheet  metal 
well  shellacked  before  assembling,  then  the  loss  due  to  eddy 
currents  will  be  negligible.  The  arrangement  of  apparatus, 
shown  in  Fig.  185,  consists  of  a  source  of  alternating  cur- 
rent, a  wattmeter,  an  alternating  current  ammeter,  an  alter- 
nating current  voltmeter,  and  the  test  ring,  all  connected 
as  shown. 


Fig.  185. 

Let  R  —  the  resistance  of  the  coil  on  the  test  ring  ; 
n  =  number  of  turns  in  this  coil  ; 
W  =  the  watts  indicated  by  the  wattmeter  ; 
/=  the  current  indicated  by  the  ammeter; 
E  =  the  pressure  indicated  by  the  voltmeter  ; 
V=  the  volume  of  the  iron  in  cubic  centimeters; 
A  =  the  area  of  a  radial  cross-section  in  square  centi- 
meters ;  then,  assuming  the  current  to  be  sinus- 
oidal, and  of  frequency  y  cycles  per  second, 


Vf 


260 


DYNAMO   ELECTRIC   MACHINERY. 


E  wing's  machine  for  hysteresis  tests  is  shown  in  Fig.  186. 
Its  chief  advantage  lies  in  the  fact  that  the  test  piece  needs 
to  consist  of  but  half  a  dozen  pieces  of  sheet  iron  f "  by  3". 
This  test  piece  is  made  to  rotate  be- 
tween the  poles  of  a  permanent  mag- 
net, which  is  mounted  on  knife  edges 
on  an  axis  coincident  with  the  axis  of 
revolution  of  the  test  piece.  The  re- 
sulting angular  displacement  of  the 
magnet,  as  marked  by  a  pointer  on  a 
divided  scale,  is  proportional  to  the 
hysteresis  loss  in  the  specimen.  A 
calibration  curve  is  plotted  by  using 
two  different  specimens  having  known 
hysteretic  constants.  It  is  found  that 
small  variations  in  the  thickness  of  the  test  piece  do  not 
affect  the  results,  and  that  no  correction  need  be  made 
for  such  variations.  The  machine  yields  but  comparative 
results. 


Fig.  186. 


125.  Determination  of  Leakage  Coefficient.  —  The  ratio 
of  the  total  flux  generated  by  a  field  magnet  to  the  flux 
passing  through  the  armature,  that  is,  the  leakage  coefficient, 
which  is  always  greater  than  unity,  may  be  found  with  an 
arrangement  of  apparatus  as  shown  in  Fig.  187  where  the 
machine  is  a  yoke-wound  bipolar.  A  test  coil  of  a  few 
turns  is  passed  around  the  center  of  the  field  magnet,  and 
through  it  all  the  lines  generated  may  reasonably  be  as- 
sumed to  pass.  A  similar  coil  is  passed  around  the  arma- 
ture, in  a  plane  perpendicular  to  the  direction  of  the  flux, 
through  which  all  the  armature  flux  must  pass.  In  the 
case  of  a  small  machine,  normal  exciting  current  is  passed 


TESTS. 


261 


through  the  field  magnet,  with  arrangements  for  rapid  com- 
mutation. In  this  case,  if  one  test  coil  have  its  ends 
attached  to  a  galvanometer  or  a  low-voltage  voltmeter,  and 
if  the  current  in  the  field  coil  be  commutated,  a  deflection, 
which  is  proportional  to  the  change  of  flux,  will  be  observed. 
The  same  will  happen  if  the  other  coil  have  its  terminals 
connected  to  the  galvan- 


If Bf  and  Oa 


ometer. 

the    deflections 

with    the    field 

armature    test    coils 

spectively,    then,    as 

fore, 


be 

observed 
and    the 


2  n ' 


re- 
be- 


and 


2  ;/„ 


k6a. 


Fig.  187. 


If  the  two  test  coils  be  constructed  alike  as  regards  number 
of  turns  and  resistance,  then  the  values  of  R,  n#  and  k  are 
the  same  in  both  equations,  and  we  have  the  leakage  coef- 
ficient 


Hence  the  ratio  of  the  galvanometer  throws  gives  the  co- 
efficient without  further  calculation. 

This  method  may  be  employed  to  obtain  the  flux  in  any 
part  of  the  magnetic  circuit,  and  it  serves  to  locate  the 
points  of  greatest  leakage.  It  may  also  be  modified  to  apply 
to  any  type  of  machine.  In  the  case  of  large  machines, 
whose  field  currents  cannot  be  commutated,  a  cyclic  in- 
crease and  decrease  of  exciting  current  can  be  produced 
by  means  of  cutting  out  and  in  of  resistance  in  the  field 


262 


DYNAMO   ELECTRIC   MACHINERY. 


circuit.  Even  then  the  time  constants  of  large  field  coils 
are  so  great  as  compared  with  the  period  of  swing  of  ballis- 
tic galvanometers,  that  the  method  is  impracticable. 

126.  Magnetic  Distribution  in  the  Air  Gap.  —  Since  ar- 
mature reactions  distort  the  magnetic  field,  it  is  desirable 
to  know  the  actual  distribution  of  the  flux.  This  may  be 


666  6  664 


LOAD 

Fig.  188. 


determined  by  the  use  of  a  pilot  brush,  as  shown  in  Fig. 
1 88.  A  voltmeter  is  connected  between  one  of  the  main 
brushes  and  the  pilot  brush,  and  the  latter  is  moved 
through  equal  angular  intervals  until  the  opposite  brush 
is  reached.  The  difference  in  the  voltage  of  any  two 
consecutive  readings  is  proportional  to  the  magnetic  flux 
within  the  angular  distance  moved  over  between  those 
two  readings. 

Two  pilot  brushes  may  be  used  as  in  Fig.  1 89.  In  this 
case  the  voltage  is  proportional  to  the  flux  corresponding 
to  the  angular  distance  between  the  two  brushes.  By 


TESTS. 


263 


applying  these  brushes   at    successive    intervals    through 
1  80°  the  flux  distribution  can  be  determined. 

127.  Measurement  of  Resistance  —  a.  By  voltmeter  alone. 
For  insulation  resistances,  or  any  resistances  lying  between 
about  5000  and  100,000  ohms,  a  fairly  accurate  result  may 
be  obtained  by  arranging  the  unknown  resistance  x  and  a 
0-150  voltmeter  in  series  with  a  source  of  constant  potential 
of  about  115  volts.  The  reading  0  is  noted.  The  resist- 
ance is  then  short-circuited  and  the  deflection  &  noted.  If 
R  be  the  resistance  of  the  voltmeter,  then 


Maximum  accuracy  is  obtained  when  x  —  R. 


AWIAMWI 

*-^ 


l 


Fig.  igo. 


Fig.  191. 


b.  By  the  Method  of  Wheatstone's  Bridge.  If  an  un- 
known resistance  :r,  two  known  resistances  a  and  b,  and 
a  known  adjustable  resistance  R  be  connected  as  shown 
in  Fig.  191,  with  a  galvanometer  G  and  a  battery  cell  B, 
a  Wheatstone's  bridge  is  formed ;  and,  if  the  resistance  R 
be  so  manipulated  as  to  prevent  a  flow  of  current  through 
the  galvanometer,  then  the  following  relation  is  true ; 

a  \b\\R\x. 


264 


DYNAMO  ELECTRIC   MACHINERY. 


It  is  usual  to  make  the  ratio  a  :  b  equal  to  some  multiple 
or  submultiple  of  10.  In  this  case  the  value  of  X  is  read 
directly  from  R  with  the  decimal  point  suitably  placed. 

This  method  permits  of  great  accuracy. 

C.  By  ammeter  and  voltmeter.  Resistances  of  ordinary 
magnitudes  are  most  conveniently  measured  by  measuring 
the  pressure  impressed  on  the  resistance  and  the  current 
caused  to  flow  thereby.  This  is  the  most  practical  method 
for  finding  the  resistances  of  armature  and  field-windings 
of  dynamos. 

It  is  a  method  so  rapid  that  the  value  of  hot  re- 
sistances may  be  found,  and  fields  can  be  measured  even 


Fig.  192. 

while  the  machine  is  in  operation.  Fig.  192  shows  an 
arrangement  of  apparatus  for  measuring  the  resistance  of 
an  armature,  including  the  brush  and  contact  resistances. 
If  /  be  the  ammeter  reading,  and  E  be  the  voltmeter  read- 
ing, then  by  Ohm's  law 


128.  Test  of  Dielectric  Strength  --  In  order  to  test  the 
voltage  necessary  to  break  down  a  sample  sheet  of  insulat- 
ing material,  the  sample  is  placed  between  two  flat  metal- 
lic surfaces,  which  are  connected  respectively  with  the  two 
terminals  of  a  high-voltage  transformer,  whose  voltage  can 
be  varied  at  will.  An  air  gap  between  needle-point  ter- 


TESTS. 


265 


minals  which  can  be  adjusted  in  length  is  connected  in 
parallel  between  the  two  terminals.  The  distance  between 
these  points  serves  to  limit  the  voltage  which  can  be  im- 
pressed upon  the  conductors  on  each  side  of  the  insulating 
material.  For  small  variations  of  gap  length  the  voltage 
necessary  to  produce  an  arc  between  the  needle-points  is 


^- 



,^- 

^ 

^ 

x*" 

t 

^ 

/ 

I 

<lf 

S'' 

2 

6 

2 

8 

3 

•3 

I 

3 

1 

3 

G 

3 

8     4 

x 

^ 

. 

,x 

__ 

^ 

^x 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

t 

/ 

/ 

f 

2J 

6        .2        4       .6       .1 

1.      1.2     1.1     1.6      l.S      2 

Fig.  193- 

nearly  proportional  to  the  length.  The  following  table, 
taken  from  the  Standardization  Report  of  the  American  In- 
stitute of  Electrical  Engineers,  shows  the  relation  which 
exists  between  air-gap  length  and  the  voltage  necessary 
to  produce  a  disruptive  discharge.  The  relations  are  also 
exhibited  in  the  curve  of  Fig.  193. 


266 


DYNAMO   ELECTRIC   MACHINERY. 


TABLE    OF   SPARKING   DISTANCES    IN   AIR   BETWEEN 
OPPOSED    SHARP   NEEDLE-POINTS,    FOR   VARI- 
OUS   EFFECTIVE    SINUSOIDAL   VOLTAGES, 
IN   INCHES    AND   IN    CENTIMETERS. 


KlLOVOLTS. 

DISTANCE. 

KlLOVOLTS. 

DISTANCE. 

Sq.  Root  of 
Mean  Square. 

Inches. 

Cms. 

Sq.  Root  of 
Mean  Square. 

Inches. 

Cms. 

5 

0.225 

0.57 

60 

4.65 

1  1.8 

10 

0.47 

I.I9 

70 

5.85 

14.9 

15 

0.725 

1.84 

80 

7-1 

1  8.0 

20 

I.O 

2-54 

90 

8-35 

21.2 

25 

1-3 

3-3 

IOO 

9.6 

24.4 

30 

1.625 

4.1 

1  10 

10.75 

27-3 

35 

2.O 

5-1 

120 

11.85 

3O.I 

40 

2-45 

6.2 

130 

12-95 

32.9 

45 

2-95 

7-5 

I4O 

13-95 

35-4 

50 

3-55 

9.0 

150 

15.0 

38.1 

In  carrying  out  the  test,  the  needle-points  are  adjusted 
at  a  certain  minimum  distance  apart.  The  voltage  im- 
pressed upon  the  terminals  is  raised  until  a  spark  passes 
between  the  points.  The  air  gap  is  then  increased  in 
length,  and  the  operation  repeated  until  the  sample  breaks 
down,  and  the  spark  passes  through  it  instead  of  across 
the  air  gap.  The  break-down  voltage  is  then  taken  from 
the  table  or  curve  corresponding  to  the  last  position  of  the 
needle-points. 

The  sample  should  project  considerably  beyond  the 
edges  of  the  compressing  surfaces.  Owing  to  surface 
leakage  a  spark  will  pass  over  a  very  much  greater  distance 
of  the  surface  of  an  insulator  than  it  will  in  free  air. 

For  the  purpose  of  obtaining  a  voltage  any  form  of  high- 
potential  transformer  may  be  used,  the  primary  being  sup- 


TESTS. 


267 


plied  by  an  alternating  current.  Fig.  194  illustrates  a 
10,000  volt  transformer  manufactured  for  this  purpose  by 
the  General  Electric  Co.  Its  core  is  of  the  H  type,  and 


Fig.  194. 


upon  one  branch  of  it  is  wound  the  low-tension  circuit, 
while  upon  the  other  is  wound  the  secondary,  consisting  of 
four  coils,  each  wound  and  insulated  independently.  The 
four  coils  are  assembled  upon  a  sleeve  of  heavy  insulating 


268 


DYNAMO  ELECTRIC  MACHINERY. 


material.  The  transformer  is  immersed  in  oil,  and  its 
primary  is  wound  in  two  parts  so  that  it  may  be  used  upon 
a  52  or  a  104  volt  circuit.  The  adaptation  to  either  of 
these  circuits  is  rendered  possible  by  means  of  a  porcelain 
series  multiple  connection  board  which  is  placed  inside  the 
inclosing  case.  On  the  top  of  the  apparatus  is  a  box  with 
a  glass  window,  which  incloses  a  micrometer  spark  gap, 
which  is  connected  in  shunt  across  the  high-potential 


Fig.    195. 

terminals.     This  box  or  cover  carries 
four  long  contact  studs  which  fit  into 
sockets.     In  the  transformer  box  the 
apparatus  is  so  arranged  that  the  lifting 
up  of  this  cover  for  the  purpose  of  ad- 
justing the  spark  gap  entirely  discon- 
nects the  spark  gap  from  the  high-potential  circuit.      The 
connections  of  this  apparatus  to  a  sample  under  test  are 
shown  in  Fig.  195. 

This  apparatus  may  also  be  employed  in  determining 
whether  a  given  sample  of  insulation  will  withstand  an  im- 
pressed electromotive  force  without  breaking  down.  The 


TESTS. 


269 


length  of  the  gap  is  set  so  as  to  represent  the  value  of  the 
prescribed  electromotive  force,  and  the  sample  is  subjected 
to  the  pressure  which  maintains  a  spark  across  the  gap. 
In  case  of  break-down  the  spark  at  the  gap  will  cease. 

In  case  it  is  desired  to  test  the  dielectric  strength  of  the 
sample  at  some  other  than  normal  temperature,  the  sample 
may  be  pressed  between  two  surfaces  of  the  apparatus 
shown  in  Fig.  196,  which  was  described  by  Mr.  Charles  F. 


Fig.  196. 

Scott.  Two  carefully  faced  blocks  of  cast  iron  are  re- 
cessed so  as  to  receive  coils  D  and  D'  of  asbestos-wound 
wire.  These  coils  are  supplied  with  alternating  current 
which  raises  the  temperature  of  the  disks  by  means  of 
eddy  currents  and  hysteresis  losses.  Upon  shutting  off 
the  current,  the  disks  and  insulating  material  soon  assume 
a  uniform  temperature,  which  can  be  measured  by  means 
of  a  thermometer  whose  bulb  is  inserted  in  a  hole  in  the 
upper  disk.  The  two  disks  are  made  the  terminals  of  the 
high-tension  circuit.  Connections  with  the  circuit  which 
is  used  for  heating  purposes  must  of  course  be  removed 
during  the  test. 


270  DYNAMO  ELECTRIC  MACHINERY. 

The  report  of  the  committee  on  standardization  of  the 
American  Institute  of  Electrical  Engineers  gives  the  fol- 
lowing :  "  The  dielectric  strength  or  resistance  to  rup- 
ture should  be  determined  by  a  continued  application  of  an 
alternating  E.M.F.  for  five  minutes/' 

"  The  test  for  dielectric  strength  should  be  made  with 
the  completely  assembled  apparatus  and  not  with  its  indi- 
vidual parts,  and  the  voltage  should  be  applied  as  fol- 
lows :  ist,  Between  electric  circuits  and  surrounding 
conducting  material,  and  2d,  between  adjacent  electric  cir- 
cuits where  such  exist." 

The  report  further  recommends  for  apparatus,  not  in- 
cluding switchboards  and  transmission  lines,  the  following 
testing  voltages :  — 

RATED  TERMINAL  VOLTAGE.  CAPACITY.       TESTING  VOLTAGE. 

Not  exceeding  400  volts Under  10  K.W.  .  .  1000  volts. 

Not  exceeding  400  volts 10  K.W.  and  over  .  1500  volts. 

400  and  over,  but  less  than    200  volts  Under  10  K.W.  .     .  1500  volts. 

400  and  over,  but  less  than    800  volts  10  K.W.  and  over    .  2000  volts. 

800  and  over,  but  less  than  1 200  volts  Any 3500  volts. 

1 200  and  over,  but  less  than  2500  volts  Any 5000  volts. 

(  Double  the  normal 

2500  and  over Any      .     .       < 

(      rated  voltages. 

Synchronous  motor  fields  and  fields  of  converters  started 

from  the  alternating  current  side 5000  volts. 

The  values  in  the  table  above  are  effective  values,  or  square  roots  of 
mean  square  reduced  to  a  sine  wave  of  E.M.F. 

When  machines  or  apparatus  are  to  be  operated  in  series,  so  as  to  em- 
ploy the  sum  of  their  separate  E.M.F?*,  the  voltage  should  be  referred  to 
this  sum,  except  where  the  frames  of  the  machines  are  separately  insulated 
both  from  ground  and  from  each  other. 

129.    Determination  of  the  Magnetization  Curve  of  a 

Shunt-Dynamo To  find  the  relation  between  the  exciting 

current  and  the  no-load  terminal  volts  of  a  shunt  machine, 


TESTS. 


2/1 


excite  tne  shunt  fields,  Fig.  197,  from  an  external  source, 
first  passing  the  current  through  a  variable  resistance  and 


Fig.  197. 


an  ammeter.  Run  the  machine  at  a  constant  speed  through- 
out the  test.  If  a  voltmeter  be  placed  across  the  armature 
terminals  a  pressure  can  be  read  corresponding  to  each 
exciting  current,  and  a  curve  can  be  plotted  using  volts  as 
ordinates  and  amperes  as  abscissae.  Because  of  residual 


HELD  CURRENT 


Fig.  198. 

magnetism  there  are  some  volts  with  no  exciting  current, 
and  hence  the  curve,  Fig.  198,  does  not  pass  through  the 
origin. 

If  the  voltmeter  be  read  while  the  current  is  increasing 
by  steps  to  the  maximum,  and  again  while  the  current  is. 


2/2  DYNAMO   ELECTRIC  MACHINERY. 

decreasing,  step  by  step,  the  two  curves  will  not  coincide  ; 
the  descending  curve  will  lie  above  the  other  as  in  Fig.  1 99. 
This  is  because  of  the  hysteresis  or  magnetic  retentivity  of 
the  iron  of  the  magnetic  circuit. 


Fig.  199. 

130.  Efficiency  of  Dynamos  and  Motors — The  efficiency 
of  these  machines  can  be  determined  by  any  one  of  the 
following  methods :  — 

a.  Run  the  machine  at  its  proper  speed  as  a  separately 
excited  motor.  Let  the  excitation  be  normal.  By  means 
of  ammeter  and  voltmeter  readings  determine  the  electrical 
input,  the  motor  having  no  load  upon  it.  The  arrange- 
ment of  apparatus  is  shown  in  Fig.  200.  The  power  put 
in  represents  the  PR  losses  in  the  armature  and  the  field 
plus  the  losses  which  are  generally  considered  as  constant 
at  al1  loads.  These  constant  losses  are  those  due  to  fric- 
tion, hysteresis,  Foucault  currents,  and  windage.  They 
are  equal  to  the  no-load  input  minus  the  no-load  PR  arma- 
ture and  field  losses.  The  PR  losses  can  be  calculated  at 


TESTS. 


273 


any  useful  load.     The  efficiency  at  that  load  is  equal  to 
the  load  divided  by  the  load  plus  the  sum  of  the  constant 


SERVICE  MAIN 


ADJUSTABLE 
RESISTANCE 


—  SERVICE  MAIN 

Fig.  200. 

losses  and  the  load  PR  losses.  The  machine  at  the  time 
of  no-load  test  should  have  the  same  temperature  as  it 
would  have  under  the  load  for  which  the  efficiency  is  being 
calculated. 

b.  Run  the  machine  as  a  motor  at  its 
rated  speed  and  temperature.  Measure 
the  electrical  input  by  a  voltmeter  and  an 
ammeter.  Measure  the  mechanical  output 
by  a  Prony  brake.  Then  the  efficiency, 

_  watts  at  brake 
watts  input 

There  are  many  kinds  of  brake  or  ab- 
sorption dynamometers  that  may  be  used 
for  this  test.  The  most  satisfactory  one 
for  motors  of  small  size  is  the  strap-brake  shown  in 
Fig.  20 1.  A  piece  of  leather  belting  and  two  spring 
balances  are  all  that  is  necessary.  The  formula  for  the 

absorbed  power  is, 

2TrrV(P-P'} 

Watts  =  ^—     — •  X  746, 

33000 

where  r=  the  radius  of  the  pulley  in  feet ; 


Fig.  201. 


2/4  DYNAMO   ELECTRIC   MACHINERY. 

V=  number  of  revolutions  per  minute  ; 
(P  —  P')  =  the  difference  of   the    two-scale  readings    in 
pounds. 

Fig.  202  shows  a  form 
of  brake  applicable  to  lar- 
ger machines.  The  for- 
mula for  the  power  ab- 
sorbed is, 

2  irrVP 

Watts  =  -         -  X  746, 
33000 

where  r  is  the  perpendicular  distance  from  the  center  of 
the  pulley  to  the  line  of  action  of  the  scale  in  feet,  P  the 
scale  reading  in  pounds,  and  V  the  number  of  revolutions 
per  minute.  The  brake  should  be  so  poised  as  to  give  no 
reading  on  the  spring  at  no  load. 

The  brake  may  be  made  with  a  metal  strap  having 
spaced  blocks  on  its  under  surface  that  screw  down  against 
the  wheel,  and  for  the  spring  balance  one  may  use  a  plat- 
form scale  having  a  prop  extending  to  the  lever  arm  of  the 
brake.  For  large  machines  the  heat  generated  by  the 
absorption  of  considerable  power  at  the  face  of  the  pulley 
causes  an  excessive  rise  of  temperature.  It  is  necessary 
to  find  some  means  of  carrying  the  heat  away.  This  is 
generally  accomplished  by  flanging  the  inside  of  the  brake- 
wheel,  forming  a  trough  in  which  water  is  kept  running. 
Centrifugal  force  throws  the  water  against  the  internal 
circumference  of  the  wheel  and  prevents  spilling.  The 
water  is  removed  either  by  a  properly  placed  scoop,  or  it 
may  be  allowed  to  boil  out. 

c.  A  convenient  method  of  finding  and  separating  the 
losses  of  a  machine  is  one  which  makes  use  of  a  rated 


TESTS.  275 

motor,  i.e.,  a  motor  whose  mechanical  output  is  known  for 
any  given  electrical  input.  By  reading  the  volt-ampere 
input  of  the  motor  the  power  expended  on  the  machine  to 
be  tested  can  be  found.  Run  the  machine  by  the  rated 
motor  at  the  proper  speed.  If  the  brushes  be  removed 
from  the  machine,  and  no  current  be  flowing  in  the  field 
coils,  then  the  power  expended  on  it  is  the  loss  due  to 
friction  at  the  bearings  and  to  windage.  Now  let  the 
brushes  be  set,  then  the  power  expended  is  the  loss  due  to 
windage,  bearing  friction,  and  brush  friction.  By  sub- 
traction the  brush-friction  loss  is  found.  This  is  greater, 
particularly  in  small  machines,  than  is  generally  supposed. 
Now  let  the  fields  be  separately  excited  by  the  normal 
current,  and  the  losses  due  to  hysteresis  and  eddy  currents 
are  included  in  the  power  expended  on  the  machine. 
From  a  knowledge  of  the  hot  resistances  of  the  machine, 
one  can  calculate  the  I2R  loss  for  any  useful  load  in  both 
armature  and  field  windings.  This  useful  load  divided  by 
the  sum  of  the  useful  load  and  all  the  losses,  gives  the 
efficiency  of  the  machine  at  that  load. 

d.  The  methods  a,  b,  and  c  all  require  some  outside 
electric  power.  This  requirement  can  be  avoided  by  the 
use  of  a  transmission  dynamometer  to  measure  the  power 
input  of  any  machine,  and  the  power  output  can  be  read  by 
a  voltmeter  and  an  ammeter.  This  method  is  seldom  re- 
sorted to,  since  transmission  dynamometers  are  often  unre- 
liable, they  are  expensive  to  set  up,  and  some  forms  have 
but  a  limited  power  range. 

Professor  Goldsborough  has  recently  devised  a  very 
ingenious  dynamometer  which  consists  simply  of  a  coiled 
or  helical  spring  with  the  center  line  of  the  helix  corre- 
sponding with  the  center  of  the  shaft.  This  spring  con- 


2/6  DYNAMO   ELECTRIC  MACHINERY. 

nects  the  driving  and  driven  members.  Readings  are  made 
by  means  of  two  instantaneous  contact  points  mounted, 
one  on  the  driven  and  one  on  the  driving-shaft,  which  are 
connected  in  series  with  each  other  and  with  a  battery 
and  telephone  receiver.  As  the  spring  becomes  deflected 
by  a  load,  the  contact  on  the  driven  shaft  falls  back, 
and  the  corresponding  brush  must  be  set  back  by  the 
same  angle  in  order  to  obtain  a  click  in  the  telephone. 
This  angle  is  a  direct  measurement  of  the  torque,  and  can 
be  calibrated  at  standstill. 

e.  The  efficiency  of  direct  connected  units  can  be  found 
by  using  the  indicator  card  from  the  steam-engine  to  deter- 
mine the  power  input,  and  by  using  a  voltmeter  and  an 
ammeter  for  determining  the  electrical  output.  Even  if 
the  engine  losses  were  exactly  known,  the  measurements 
yielded  by  an  indicator  card  are  hardly  exact  enough  to 
afford  a  fair  basis  for  testing  the  efficiency  of  a  generator. 
In  other  than  direct-connected  units  it  is  not  frequent  that 
one  finds  a  generator  driven  by  an  engine  that  does  no 
other  work. 


INDEX. 


Air-gap,  69,  8 1,  85. 

distribution  in,  262,  87. 
Ampere-turns,  21,  245. 
Angle  of  lag  or  lead,  82. 
Armatures,  34,  47,  51. 

Back-turns,  82. 

Bearings,  64. 

Boosters,  216. 

Box,  starting  for  motors,  169. 

Brake  on  motors,  179. 

prony,  273. 

solenoid,  182. 

strap,  1 80. 
Brushes,  60,  61,  86,  90. 

Candle-power  of  arc  lamps,  130. 
of  incandescent  lamps,  103. 
Capacity  of  a  dynamo,  36. 
Cauteries  for  surgeons,  215. 
Chord  winding,  52. 
Circuits,  divided,  7,  8. 
Coefficient  : 

economic,  93. 

of  series  dynamo,  97. 
of  compound  dynamo,  1 1 1 . 
of  shunt  dynamo,  100. 
of  conversion,  93. 
of  magnetic  leakage,  73,  260. 
of   self   and  mutual   induction, 

17,  18. 
of  temperature,  5. 


Coercivity,  28. 
Coil: 

armature,  46,  49. 

compound,  71,  246. 

field,  71. 

formed,  55. 

Collection  of  currents,  61. 
Commutation,  84. 
Commutator : 

construction  of,  59. 

losses  at,  59. 

principle  of,  32,  77. 

segments  of,  33. 
Compounding : 

in  motor,  174,  224. 

windings  for,  71,  in. 
Conductivity,  5. 

of  copper,  6. 
Connections : 

for  combined  output,  218. 

of  motors,  223. 

series  dynamos  in  series,  221. 

shunt  dynamos  in  parallel,  220. 

shunt  dynamos  in  series,  221. 

cross  in  armature,  48. 
Constant : 

determination    of    ballistic    by 
condenser,  254. 

determination    of    ballistic    by 
long  solenoid,  255. 

determination  of  ballistic  earth's 
field,  256. 


277 


2/8 


INDEX. 


Constant : 

determination  of  hysteretic,  256, 
259,  260. 

hysteretic,  29. 
Contact  of  brushes,  60. 
Controller : 

for  mill  motors,  206. 

for  street-railway  motors  197. 

fingers  for,  198. 

reversing  lever,  199. 

wipes,  198. 
Core,  armature,  37,  57. 

field,  67. 

Correction  for  tooth  densities,  243, 
Cross-magnetization.  80. 
Current  density,  72. 

in  armature,  234. 

Currents,  foucault  or  eddy,  38,  90. 
Curve,  B-H.,  24,  25,  244. 

characteristic,  96,  98,  101,  174. 

determination  of  B-H,  251. 

determinization   of    magnetiza- 
tion, 270. 

Demagnetization,  82,  246. 
Density  of  flux,  22. 

in  air-gap,  233. 

corrections  for,  in  teeth,  243. 

determination    of     distribution 
of,  262. 

in  field,  131,  241. 
Design,  data  sheet  for,  247. 

different  methods  of,  232. 

of  armature,  235. 

of  field,  241. 

preliminary  assumption  for,  233 

specifications  for,  233. 
Diameter  of  armature,  237. 

of  shaft,  64,  119. 
Dielectric  test  of  insulation  of,  268. 

test  of  strength  of,  264,  270. 


Direction  of  induced  E.  M.  F.,  16. 

of  rotation  of  a  motor,  161. 
Drop,  4. 
Drums,  34,  51. 
Dynamo  : 

arc  lighting,  129. 
Brush,  134. 
Westinghouse,  141. 
Wood,  143. 
Excelsior,  147. 
Ball,  149. 

Thomson-Houston,  151. 
Western  Electric  Co.,  156. 
definition  of,  31. 
direct-driven,  118. 

Bullock  Electric  Co.,  127. 
Crocker-Wheeler,  123. 
General  Electric  Co.,  121. 
Lundell,  88,  124. 
Sprague  Electric  Co.,  88, 124. 
Westinghouse  Co.,  119. 
Dynamotors,  208. 

armature,  reaction  in,  208. 
for  Bullock  teazer  system,  209. 
for  electrometallurgy,  211. 
as  rotary  equalizer,  212. 
for  telegraphic  work,  213. 
Dyne,  definition  of,  i. 

Efficiency,  92. 

of  compound  dynamo,  112. 

of  compound  motor,  165. 

determination  of,  272. 

of  direct  connected  units,  276. 

of  motors  for  automobiles,  202. 

of  shunt  motors,  166. 

of  series  motors,  166. 
E.  M.  F. :  constant    supply   of, 
103. 

counter  in  motors,  163. 

direction  of  induced,  16. 


INDEX. 


279 


E.  M.  F.  : 

in  shunt  dynamos,  100. 

in  separately  excited  dynamos, 

95- 

in  series  dynamos,  97. 

of  induction,  14. 

of   self    and  mutual  induction, 
17,  79,  84. 

of  eddy  currents,  38. 

principle   of   production   of,  in 
armature,  32. 

unit  of,  3. 
Energy,  i,  89. 
Equalizer,  bus,  222. 

rotary,  212. 
Erg,  i. 
Excitation,  mutual,  222. 

separate,  94. 

of  fields,  69. 

Fall,  of  potential,  4. 
Feeding-points,  104. 
Field,  magnetic,  12,  21,  67. 
Fleming's  rule,  16. 
Fluctuation  of  E.  M.  F.,  34. 
Force,  magnetizing,  22. 

magnetomotive,  20,  26,  245. 

units  of,  i. 
Foot-pound,  i. 

Frequency  of  commutation,  84. 
Friction  of  bearings,  89. 

of  brushes,  60. 
Fuses,  10. 

Gap,  air,  69,  81,  85,  233. 
Generators  [see  dynamo],  31. 

Heat  of  current,  9. 
Heating  of  armatures,  39. 
Holders  for  brushes,  61. 
Hysteresis,  27. 


Hysteresis : 

losses  by,  89. 

Inductance,  17,  18,  84. 
Induction: 

electro-magnetic,  13. 

mutual,  1 8. 

self,  17,  79. 
Inductors,  34,  45. 
Input,  92. 
Intensity  of  magnetic  field,  12. 

Jig,  for  filing  brushes,  62. 
Joints  in  magnetic  circuit,  76. 
Joule,  definition  of,  i. 

Lag,  82. 

of  brushes  in  a  motor,  165. 
Lamination,  38. 
Law  of  Steinmetz,  29. 
Lead,  82. 
Leakage,  magnetic,  73. 

determination  of,  260. 

in  compound  motor,  175. 
Length  of  armature,  237. 

of  active  conductor,  235. 
Lines  of  force,  12. 
Link,  fuse,  10. 
Losses,  92,  93. 

armature,  238. 

commutator,  59. 

fixed,  167. 

I2R,  9. 

in  operation,  89. 

variable,  167. 
Lubrication,  65. 

Magnet,  field,  35,  69. 
Magnets,  70,  96. 
Materials,  insulating,  6. 
magnetic,  24,  68. 


280 


INDEX. 


Matthiessen,  standard  of,  6. 
Measurement  of  temperature  rise,  42. 
Melting  of  commutator  bars,  83. 
Meters,  recording  watt-hour,  182. 
Mica,  7,  59. 
Mil,  circular,  6. 

-foot,  6. 
Motor,  brake,  179. 

compound  wound,  174. 

counter  E.  M.  F.  of,  163. 

direction  of  rotation  of,  161. 

for  railways,  187. 

for  automobiles,  200. 

for  mills,  203. 

principle  of,  161. 

rated,  274, 

series,  185. 
Motor-generators,  215. 

Oilers,  65. 

Operation,  care  in,  225. 
Output,  92. 

Over-compounding,  in. 
coils  for,  246. 

Permeability,  22. 
Permeameter,  253. 
Permeance,  25. 
Plane : 

commutating,  78. 

neutral,  78. 

Point,  running  on  controller,  197 
Pole,  magnetic,  12. 
Pole  pieces,  67,  75. 
Potential,  magnetic,  19. 
Power,  lines  of,  99. 

of  electric  current,  8. 

unit  of,  2. 
Pressure,  4. 

Prevention  of  sparking,  85. 
Process  of  commutation,  77. 


Rating  of  machines,  39. 
Reaction  of  dynamo  armature,  80. 

of  dynamotor  armature,  208. 

of  motor  armature,  165. 

Rectifier,  compounding,  112. 
Re-entrancy,  46. 
Regulation,  arc  dynamo,  132. 

hand,  104. 

self,  110-113. 

(see  speed). 
Reluctance,  25. 

of  dynamo-magnetic  circuit,  242. 
Reluctivity,  26. 

Report  of  Standardization  Commit- 
tee of  the  American  Insti- 
tute of  Electrical  Engineers : 

on  efficiency,  92. 

on  regulation,  116. 

on  spark-gap  voltages,  266, 

on  temperature  elevation,  40. 

on   testing    dielectric  strength, 

270. 
Resistance,  armature,  240. 

brush  contact,  60,  90. 

measurement  of,  263. 
Resistivity,  5. 
Retentivity,  28. 
Rheostats,  Carpenter  enamel,  108. 

overload,  172. 

packed  card,  105. 

starting,  169. 

Ward   Leonard    Electric     Co., 
108. 

Wirt,  no. 

Rise  of  temperature,  41,  91,  60. 
Rockers,  61,  64. 
Rule,  Fleming's,  16. 

Saturation  of  teeth,  85. 
Shafts,  64. 


INDEX. 


28l 


Shape  of  pole  pieces,  86, 

Sheet,  data  for  design,  247. 

Shell,  magnetic,  20,  21. 

Shifting  of  brushes,  85. 

Shoe,  pole,  75. 

Short-circuit  of  armature  coil,  226. 

Skewing  of  field,  81. 

Slip-rings,  31. 

Slotting  of  poles,  86. 

Solenoid,  21. 

brake,  180. 
Span,  polar,  69. 
Spark,  voltage  of,  266. 
Sparking,  61,  83,  85,  225, 
Spectrum,  magnetic,  13. 
Speed,  armature,  239. 

motor,  163. 

slow,  178. 

hand  regulation  of,  175. 

Leonard  system  of  regulation, 
176. 

series  resistance  regulation,  1 76. 
Steinmetz,  law  of,  29. 
Strength,  test  of  dielectric,  264. 
Surface,  for   radiation  in  armature, 
240. 

Teazer  for  Bullock  system  of  speed 

control,  209. 
Teeth,  85. 
Temperature,  critical,  25. 

measurement  of,  42. 

rise  of,  41,  43,  60,  91. 


Tension  of  brush  springs,  62. 
Theory  of  self-regulation   of   dyna- 
mos, 113. 
Torque,  2. 

Transformer  for  high  potentials,  266. 
Turns,  series,  no. 

Unit,  absolute  and  practical,  2. 
mechanical,  i. 
of  current,  3. 
of  potential  difference,  3. 
of  resistance,  3. 
pole,  12. 

Velocity,  peripheral  of  armature,  234. 
Ventilation,  ducts  for,  39. 
Vulcabeston,  7. 

Watt,  definition  of,  2. 
Wheatstone's  bridge,  263. 
Windage,  89. 
Winding  chord,  52. 

closed-coil,  45. 

cross-connected,  48. 

drum,  51. 

open-coil,  44. 

ring,  47. 

series,  70,  97. 

short-connection,  49. 

shunt,  70,  97. 
Wires,  binding,  55. 
Work,  i,  9,  19. 

Yoke,  36,  67. 


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